Washable diapers are somewhat different from regular clothes. They need to be natural fibre (cotton or similar) that can handle being washed at high temperatures - because you want to make sure those diapers are properly clean. Natural fibre is expensive - there is a reason why most denim pants on the market are stretch now. Washable diapers also harken back to a time when one parent - usually the mother - was at home all day to look after the household and the kids. Water and electricity were cheap back then - nowadays if you’re running several loads of washing each week just to clean the diapers, you’re quickly transferring that cost.
Your other analogies are similarly flawed. Home repairs are expensive, sure - but watch a Youtube video? Really? For one, some skillsets are not transferrable through a video - these people make it look easy because they know what they’re doing. Then, even if I can comprehend what I’m supposed to do, I still may not have the required tools - and boy some of them can be expensive. Thirdly, certain kinds of work are regulated, at least where I live (plumbing, electrical), so DIYing those can get you into very hot water if something goes wrong. Lastly, if I botch a repair or break something else in the process, I’m left holding the bag. A tradesman has the required insurance - at least where I live - and has to warrant their work.