Because how many website operators continuously google their own web site on mobile devices to click through to experience their google-hosted AMP editions? I would wager it's a fairly small % of the number of websites running an auto-AMP-ifying wordpress plugin, for example.

I've seen plenty of badly designed mobile websites over the years and these sites seems to have turned out okay - most mobile browsers keep the option of "Request Desktop site" very accessible for a good reason.

The worse case I've seen is a site in which every page crashed Mobile Safari without fail regardless of which version you ask for. It was eventually fixed but I never figured out why. If the sites are just running some script without checking then the admins have failed their line of duty.