This really isn't true. One of the most famous speedrunning games is Super Metroid, and its two glitchless categories are the most actively competitive ones. Ocarina of Time glitchless had a new world record a few months back. Glitched speedruns tend to get more coverage in gaming media (because "runner completely breaks game" is clickier than "runner plays game 0:01 faster than previous runner"), and they tend to show up a lot at marathons (because they're faster), but generally speaking if you don't like seeing glitches there is plenty of content out there for you.