Entities glitch through blocks making it more difficult to build some farms with 1 thick wall because when they are cramped in a space they tend to glitch out and go through blocks. It is also difficult to build a wither cage because spawning them will either cause them to spawn outside or inside the wall glitching outside. This will be annoying when your afk on a mob farm then one mob glitches through ended up killing you. The more reasonable way is to let the wither suffocate on the cage if the hitbox is in a block and mobs cramped together in a trident killer should just bump the walls even if it's a transparent block like powered pistons and the piston head.