Actually, for a while during testing we had the random monitor odds put onto the ? monitor and we used that. Eggman was by far the most rage-inducing thing you could possibly imagine. If you think the Recycler is random and annoying you didn't play match under that system. Eggman boxes do not belong on objects designed to be beneficial.
The appeal of doing it by random respawn instead of by the ? monitor is two-fold:
1. The player can see what item they're going to get before they put forth the effort to hit the box. This means that if the player has say, an Elemental Shield, they can choose whether or not they want to hit the box for a Whirlwind Shield or Force Shield, which may actually depend on the situation and level. I'd much rather keep an Elemental Shield in Infernal Cavern, but when I go up and look at the SRM I can decide whether I want the item or not given what I have. If it was a ? icon I couldn't do that. I like giving players decisions to make in gameplay.
2. The random monitors get to look like every other monitor in the stage. WRMs are quite common, so by using random respawn they don't stick out like a sore thumb and instead just end up as whatever item they happened to spawn as this time.