That's not necessarily the case. A bar that has a bouncer all the time? It might be that they get a shitty crowd, but a lot of times it's just that the bar is too busy for the bartender to remember which patrons they've checked IDs for. The simpler solution is to have someone stand at the door to check IDs so that every one inside has been checked.

I worked at a bar that only had a guy outside on Thursday, Friday and Saturday after 9. The rest of the time it was slow enough that the bartender could just check IDs before the customer was served.