I own a website with about 50k pageviews a month. This could mean that actual viewership could be -much- higher?
It's insane that GA doesn't offer a server-side solution like SimpleAnalytics does.
It's not really on GA but rather the ad blocking plugin you use (Or even if you are using some browsers that try and block trackers).
As an example, if you use UBlock Origin, I think by default it uses the "Pete Lowe" list to block, which by the looks of it Simple Analytics is also blocked on : https://github.com/simpleanalytics/roadmap/issues/200
Typically UBlock is just a "block all" type thing. Adblock is a little different in that they started allowing through "Acceptable Ads" (Not sure if that extends to trackers), which people went absolute ape about because now "Ad Block" doesn't block "all" ads, just ones that it deems not acceptable.
But anyway. TL;DR; Any tracker, ad, CDN, hell even HTML Fingerprint can be blocked in ad blocking plugins, just depends on the will of the people maintaining the block lists to add it.