This isn't really true. Most fantasy stories are adventure/exploration and it doesn't really matter if it is or not adventure/exploration stories. If story is fantasy then you need to would build properly. If not, then why bother with the fantasy element at all? Why not make your life easier and not have Fantasy elements?
I don't get this question, why do you think it would be harder to write a story with fantasy elements than one without? It's not like they add complexity to the story just by existing.
Then ask: Do you really need the fantasy element at all? Did the magic really makes an impact in your story? Can't you set it in regular non-fantasy medieval setting?
Fantasy is convenient. It's easier to play with different races if you have fantasy, for example, in my romance novel I wanted there to be practical issues that made the romance between the protagonist and her fairy companion harder, issues that wouldn't exist if they were both humans.
Magic also allows you to skip over some processes that might take longer otherwise, like... Rather than researching the efficacy of oils that people used in their hairs in place of shampoo/conditioner back in medieval times, you can just say "Oh, they use magical oils, so it makes the hair shiny and glossy.", and you can also say the hair is dried with a spell, so you don't need to worry about rather they'd let it dry naturally or wrap a rag around it... And yes, those details can matter, and being able to find simple solutions for them is nice~
You can also use fantasy to set up differences between social classes, like saying that the royal family is blessed by the gods, so their rule is divine (thought that was also done IRL, it's nicer when you can say it and it's true). Or how you can set up that magic depends on bloodlines and someone might get jealous of someone else because they were simply born with better magical capacity... And other similar stuff.
You can find equivalents to those things in non-fantasy if you try, but fantasy gives you an array of tools you can use as you please. It's practical, and also helps control my own need of wanting to make things as historically accurate as I can. I can have some leeway of inaccuracy if I have magic compensating for it after all~
And lastly, I just like fantasy. As simple as that. I want to write fantasy because I like fantasy, I don't have any desire to write non-fantasy novels in the first place, so there is no point in me writing them.
And you can misuse your tools. Do you really need the Fantasy elements to tell your story? You can write Romance and Slice-of-Life without any Fantasy elements at all.
If you add Fantasy elements, then it has to do something with story. You need to put effort to world building or else why is it there?
Mostly answered above, but again, adding fantasy elements just means that I'm adding something I like to the story. It doesn't need a greater purpose of existence or anything along those lines.
Like uhn... Let me use an example of another genre to illustrate my point.
If I want to make a Girl's Love Story, I don't need to have a greater reason for it to be Girl's Love. I don't need to play with the fact they'll be unable to have children, nor do I need to deal with the prejudice that two girls loving one another might go through... Sometimes you can just make a Girl's Love Story because you want it to be Girl's Love.
Could it also work if it was a Heterosexual Romance? Probably. Does it need to be? Not really. And if it doesn't need to be Heterosexual, why can't I just make it Girl's Love instead? Maybe it's just a representation issue, or maybe it's just because I feel like doing it, maybe because I just like female characters more than I like male characters and I want to revolve my story around those. There really doesn't need to have a bigger reason behind it.
The same applies to fantasy, I don't need to put a lot of effort into making it unique or making a carefully crafted world with specific rules for how magic works and the whole fantasy aspect... I just want to write fantasy, so I just put the things I want and let the reader's common sense fill in the rest. Not everything in the story
needs to have a purpose of existence. Some elements can be there just because you like them there, they can add a nice charm to the story even if they aren't very relevant.
Or is it because you think that a simple Romance or Slice-of-Life story is not interesting enough hence you add Fantasy elements to pick up the slack? Really, I can't see other reasons for adding fantasy, even though it is not needed, other than as a gimmick.
Naaaaaaaaaah, I write what I like and I like what I write. I don't care about how interesting it feels to others, I'm sure there are people out there that have a similar taste to mine, so they'll like my story if they read it. I just appeal to my own tastes and make sure I have fun doing it~
Trying to add new things to your story just to see if more people will give it a shot just feels weird, that's not for me~
The problem with your argument, is that you think of adding fantasy as something that is actively strenuous and that puts a burden on the writer and makes the writing process harder than normal... But fantasy elements can just be ingrained at the story from the start, from the moment of its conception, sometimes they are just there because the story was thought of as a fantasy story from the beginning, there is no need to change that and go through the hurdle of substituting the fantasy elements for real world equivalents when you can just... Keep them there.
Rather, medi-fantasy gives you access to more tools than a medieval world story... You can play with everything that makes a medieval setting work, but you can also play with everything that makes a fantasy setting work. I really can't see how fantasy elements could actively make your life harder when you can just use them as an extra layer of tools that help make your story function.