Sometimes I wonder whether people who complain about this are generally just pretty average, upset about being upstaged by people they see as inferior and angry that some of their colleagues get more help than them, but due to their inherent averageness haven't taken the time to consider just why it is the case.
Exams are such that everybody has the potential to get full marks. It's a test of how well you know the facts, and how experienced you are in tackling the problems. Both of these, along with good marks, come with preparation. Someone who 'struggles' but doesn't qualify for extra time is a person who doesn't work hard enough. For a lot of reasons, no doubt: lack of personal motivation, (critically) lack of parental incentive, lack of understanding why a thing is important. None of these are disabilities, they are attitudes parents have permitted to persist.
Those with qualifying disabilities are people who, no matter how well they know the material and how experienced they are with tackling the problems associated with the material, could not either read the questions in sufficient time or could not get their answers down in sufficient time.
Extra time is something that helps those who can not do a thing. It's not there to help those who, for one reason or another (irrespective of who's at fault), will not.