There are two kinds of Charity shops. One kind is local, run almost entirely by volunteers, the usually for a local charity and sells also for the benefit of local people, so fairly cheaply. That’s how they started off.
Then there is the big business Charity shops. These are run entirely for profit. There is no other motive, such as the OP is suggesting like providing a cheaper alternative for people who are hard up. They operate with a hierarchy of paid staff. Manager, Assistant manager, District manager, Regional manager, Marketing manager, Head of purchasing ( a lot of the stuff you see is bought in from wholesale) Head of Retail and all the associated admin and accounting staff.
The costs are enormous and any shop that does not raise its expected target to contribute to these costs and cover it’s running expenses will be shut down.
It’s a business decision. Big name charity shops are pretty much like any big name retail organisation- except that they don’t have to pay staff at the lowest levels 😬