Old games got crazy with copy protection for a while and it's not as easy as just putting in a disc and say make iso. They are all kinds of bad sectors they put on there, all kinds of tricks that make it a pain to copy.
Also not that installing from an iso a lot of times will then involve some sort of no-cd patch.
I get you are trying to backup your legally owned games to install on a new machine, but it's more of a hassle than it's worth. They had to go to extreme lengths to copy discs back then.