Is because he has maximized his limited abilities, both in college and the NFL. Somebody with his limited physical tools and groomed in a college system that is virtually antithetical to NFL offenses should and almost without exception does fail to carve out any kind of NFL career. But he made himself into a program changer at UH and has managed to work his way into what could arguably be considered a serviceable NFL backup.
I'll take watching Keenum continue to work at defying the odds over watching pretty much any other QB the Rams have trotted out there over most of the last decade. That is, until Goff is ready.