I think you're underestimating just how much competition there is in the job market right now, even in minimum wage or entry level work. The older generation is taking longer to retire than expected, so you have a lot of overqualified young people stuck in these low level jobs until something better opens up, and every year more and more are coming out of college and taking what they can find as administrative assistants, retail workers, fast food managers, etc.
Annually, I have one entry level opening. For that one opening, I can easily get 500 resumes. That's 500 people who want the 1 job I will have available all year. I want the best of the best from that 500. Someone who can do the job I have available, fit in with the team I already have in place, mesh with the company culture, and grow into other roles. From that 500 I interview 10 people only, and I need to be 100% confident that the person I hire can do all of the things above.
Literally all 500 of those people would love a chance, and many of them need it. But my responsibility is to my company's owner, his partners, our clients, and the staff I currently have in place - not an applicant. If someone says to me before they're even hired that they aren't going to be able to meet the bare minimum, I can't in good faith hire them because it would not be a good decision for the business as a whole. It would be much different if I've already committed to this person and they come to me after being hired and tell me that they're having a rough time. I would not be happy about it, but I would try to make it work for them because obviously I thought they were a good enough fit to hire in the first place.
That is the difference. I wholeheartedly do not recommend taking what you probably see as a proactive approach in letting people know up front that you have concerns about your ability to do the work. Get someone to like you first and hire you, then try your best. If your best is not good enough, then have that conversation. By telling them up front, IMO you are sabotaging yourself.