Great question. Some BnB hotels and condo rentals do have reservation systems that let you choose the specific room. They tend to be places where each room is unique or separately owned, though.
As for why you can't pick your room in a typical hotel, I'm not in the business but I can imagine the reasons are mostly advantageous only to the hotel. Not really for malicious reasons, but just designed to maximize occupancy. I kinda imagine a Tetris type of board where, if the hotel management is allowed to control it, will be packed with no void spaces - always as close to full capacity as possible. In fact, it will be overbooked with the expectation of cancellations. If they let the visitors control it (i.e. choose your own room) then the schedule will have a lot of void spaces (unbooked days) with the best rooms taken and the less desirable ones unbooked.
There are times when I would pay a premium with no-refund allowed if I could lock in a specific room, though. I'm sure others would as well. There's probably an opportunity for a hotel to do this.
As for why you can't pick your room in a typical hotel, I'm not in the business but I can imagine the reasons are mostly advantageous only to the hotel. Not really for malicious reasons, but just designed to maximize occupancy. I kinda imagine a Tetris type of board where, if the hotel management is allowed to control it, will be packed with no void spaces - always as close to full capacity as possible. In fact, it will be overbooked with the expectation of cancellations. If they let the visitors control it (i.e. choose your own room) then the schedule will have a lot of void spaces (unbooked days) with the best rooms taken and the less desirable ones unbooked.
There are times when I would pay a premium with no-refund allowed if I could lock in a specific room, though. I'm sure others would as well. There's probably an opportunity for a hotel to do this.