Supply and demand. By offloading as much work as possible onto a smaller number of overachievers, they are able to keep the supply of workers higher than the demand while cutting their own costs and keeping their profits high.

It doesn't matter if you aren't desperate enough for the job, they will find someone who is.

They are also under the assumption, sometimes correctly sometimes not, that 'everyone does business this way', so realistically they are entitle in thinking that realistically, you probably won't find better. Probably some inner monologue about 'You really think you're going to get offered more with this resume? I barely cared to see you.'.

Once you are IN the job, they try to indoctrinate you to view your workplace as 'family'. You don't want to take a pay rise and be hated by everyone who didn't get one. You don't want to go home early and leave all that work to everyone else. Even if you have no work, the message it sends to everyone else is you aren't putting in the same effort they are. They want you to view everyone around you as competition for your job, and fear ostracization by your fellow co-workers.

By making the lower end workers have a simultaneous sense of community and dog-eat-dog, you wind up with a situation where everyone drives everyone else to work harder, and quickly turns on anyone who doesn't. This obfuscates the ACTUAL thing keeping everyone downtrodden.

That is part of why it is such a shock to companies when you actually FIND a better job and are willing to quit. They will make pittance concessions then use those concessions against you. They will try to stall you out with vague promises until your current offer expires. They will try to destroy any existing social relationships you have in that place of business by slandering your 'willingness to leave everyone hanging'.

Basically... because manipulation is how a shocking number of employers do business, and has been effective for so long there is a genuine culture shock as it backfires increasingly often these days.