I strongly dislike bed frames. I have been sleeping on a mattress on the floor for the past 6 months, and it’s much better.
You can reach anything you’ve put next to your mattress, without needing a bedside table.
Bed frames are often creaky, which can be a nuisance to yourself and others, as well as make it harder to fall asleep.
Bed frames are physically solid and can be uncomfortable. For example, I like lying with my lower half on the bed with my top half off. Doing that with my old bed frame, there was a piece of wood sticking into my ribs, which isn’t a problem with just a mattress. (edit: I don’t sleep with half my body off the bed. I lie that way when I am doing crap on my phone instead of getting up and being productive)
Bed frames sometimes cover up outlets, depending on how they’re placed.
For people who roll around when they sleep, falling off just a mattress is less painful than a raised bed.
Bed frames cost time and money to shop for, buy, and assemble.
Despite this, every person I’ve ever met finds it weird that I prefer a mattress on the floor, so I guess it qualifies as an unpopular opinion!
Edit: i did not know that your mattress needed ventilation to prevent it from growing mold. my mattress does not have any mold and i intend to keep it that way. thanks for letting me know, you all have saved me from having to spend a fortune on a new mattress