Why can't all the banks share a single premise, that way everyone can have branch banking, just not everyday.
Several reasons of the top of my head:
1) Because none of the big banks want branches any more, and if they do, they'd be in the centres of big cities and towns where they can get a lot of footfall. Marple is tiny by comparison. All the "new" banks (Mondo etc) are Internet only and have no branches at all. If you recall, First Direct did this YEARS ago as an experiment on behalf of Midland Bank (remember them?) as a telephone bank only.
2) The IT support would be unbelievably complex. Banking systems have significant requirements in terms of security, network separation, networking standards etc. The networking closet(s) would be huge and very expensive to support. The difficulty of getting the big banks to agree to this, whilst not insurmountable, would be considerable and probably not worth the ROI (to them).
3) There are alternatives to most common banking procedures, which is using a Post Office. I'd expect these services to be improved and streamlined over the next few years to accommodate corner-cases which aren't currently covered.
As an aside, I have been a NatWest customer for years, I was disappointed when it closed but I only ever went for two reasons...firstly to pay in dividend cheques in USD for which there is a £5 charge and I wait until I have enough of them to make it worthwhile. Now I just post them to a central processing centre. Secondly to use their EXCELLENT change sorters which sort all the change, give you a chit which then pays the amount straight to my account. But I cant' remember the last time I actually used cash. I think I still have a pot on the sideboard containing our team's winnings from the last Marple Sports Club quiz (March 2020!). When they open again, we'll use it to pay for drinks until we win again.
Like it or not, branch banking is more or less dead along with many other high street branches. I remember BT shops, Norweb branches etc, all of which were killed off by remote banking (direct debit etc).