They are getting paid to be there. Can you imagine going to an interview, being asked to "gasp!" actually demonstrate a task commonly performed at your job and saying, "nah dog I'm good, where the food at?".
As of 2022, players were not getting paid. I read an article 2 years ago about agents having to advance players travel money to attend. Players who attend then don't get drafted are screwed. I am unable to find if that has since changed.
The combine is not equivalent to a typical job interview. The players are offering their truly unique individual talent that cannot be replicated by anyone else. They have already spent years demonstrating the "task commonly performed at your job" and it is all on film for any prospective contract partner.
The NFL Draft System already creates a power imbalance in favor of the owners. Why should players feel any pressure to perform like trained monkeys to please any future contract partners? Players are there for interviews to find partnership fits, and have every right to spend time meeting with team representatives to find good fits and persuade those teams to 'hire' them.
They also have the right to tell teams in which they have no interest "don't draft me" for whatever reason they choose.
The combine is about a lot more than tasks and numbers. It's good to see players utilizing their leverage to tweak a system that is rigged against them in the name of "competitive balance".
If a team is interested in further observation, they can send reps to the players' home ground for their pro-day.