I think because with the Kinder Surprise you’re always purchasing pretty much the same worthless piece of plastic.
There is a vivid aftermarket for these promoted toys. They're worth more if you collect a full set.
You're right insofar as the non-promotional toys are pretty much only worthless plastic.
The ones I got still have value for me. Even some of the "worthless plastic" toys are actually quite creative (at least you often get some fun assembling them) and thus more than anything that can fall out of a digital loot box. As a child I didn't want to throw them away.
What is not coming up here very often is the irrational customer behaviour of buying digital objects, which will disappear when the servers turn off.