The trouble is that when you have to use the supplies that are provided for you, they're inevitably shit quality compared to what you can get elsewhere.
So for example, my 14 yr old DS is still in nappies/pull-ups. In theory, I could get nappies prescribed by the Continence Service on the NHS.
However, the Continence Service often don't want to provide pull-ups, only nappies. My DS is learning how to change his own pull-up. Nappies will set his independence and personal skills back. It is allegedly possibly to get pull-ups prescribed if you're willing to fight but the only brand of pull-up they supply leak and don't fit very well.
As anyone who has used nappies/pull-ups for teens or adults will testify, a good fit is absolutely vital. Not every brand will fit someone in the right way, and will cause leakage. My DS is very slim so some brands gape in the leg which is obviously an absolute disaster.
Being given money means I can try out different brands and find one that fits him properly, even if it's more expensive than the ones the NHS would provide. Hence why I haven't bothered getting any on prescription and I just buy our own.
This is just one example but it's this kind of thing that would be problematic. I can virtually guarantee that by trying to save money they'll insist on prescription only supplies for things like this which don't take any personal needs into account.
People who don't have a disability or don't care for/live with someone who has a disability don't necessarily realise that it's not as simple as "have this thing that you need" - it's sometimes necessary to shop around or find alternatives that work better. Vouchers or equipment from the government will inevitably deny disabled people the choice to get the best items.