Can you ask for a higher pay or you need someone to ask on your behalf?

Every now and then some of my friends would tell me that their employers are treating them badly. Chief reason is the employer does not wish to pay them good salaries. When I asked them why are they still working there if they feel that the employer is not treating them right? Some said they are still happy there, some say they could not find another role giving them the pay want and some say that other potential employers are also offering a similar pay and thus not worth for them to resign.