[Konon]

It is bad that there is no single standard.

agree! But we have to live with that either fanart.tv or tmdb will change its conventions or retouch it's billions content afterwards (tmdb didn't even have conventions... 2:3 aspect just got the most common)
But many don't even notice or don't care.

Pretty sure you also notice this currently more extreme cause you came into the matter more focussing on it. 

I noticed this as soon as I switched to KODI. We can say that thanks to this I began to make media content myself.