YANBU to wonder, but there are many issues that could affect someone's use of them.
They can be prohibitively expensive. Even a reasonably cheap pad will cost at least £3 and that's a lot when you can buy 12 pads from Wilko for 95p. Of course it factors out cheaper in the long term, but often poverty or living pay packet to pay packet excludes the possibility of thinking long term about products to buy.
Obviously if you don't have a washing machine at home and have to use a laundrette, this could make it more difficult as many people wouldn't be willing to take soiled pads to a launderette to wash.
I don't find them messier than disposables personally. I take mine off, stick in the sink, put a new one on, rinse the old one out and wring it out. Then I chuck it in the wash with the rest of my clothes. I wash every day because I have two mucky children and I'm fairly fastidious about having clean clothing on. As well as clumsy 😂 But for those that don't do laundry every day, it definitely could seem a lot worse.
TBH, I make my own out of whatever I can find, like the remnants bin at Fabric Land for the tops/bottoms and old muslin/nappy inserts/tea towels/pillowcases for the inners. But not everyone has access to cheap fabrics, I am lucky to have that. And not everyone has the time or ability to sew them.
To the ladies that worry their periods are too heavy, I actually find them better for heavier periods than disposables. I have very very heavy periods as I have endometriosis, ovarian cysts, adenomyosis and fibroids. I could easily go through 14 pads on my heavy days and often disposables feel moist and gross within minutes of changing. I don't get that with disposables. But they are a bit bulkier, especially for night time pads. Not especially so if they're well made, but enough that people might feel self conscious wearing them at work or whatever. I don't work out of the home so it's not an issue for me.
I wish more people would use them to save on landfill and reduce industrial pollutants. Plus they're likely better for your vulva/vagina health than disposables. But not everyone can or wants to and I respect that.