I used to get annoyed by this too but came around to accepting it after building a Web (Jekyll) photo processing pipeline of my own. Art direction is very difficult to do well when you have to be able to handle any user-uploaded photo that might be in any aspect ratio. Desktops favor displaying wide photos, and handsets favor displaying tall ones. Cropping everything down to a square lets you handle any combination of photo and display aspect ratios with minimal Badness.

I’ve never seen the Twitter-dot-com codebase but would be willing to bet it’s probably using VIPS’ smartcrop feature (:crop => :attention) https://www.rubydoc.info/gems/ruby-vips/Vips/Interesting