Well, silent image also includes creatures and visible phenomenon. Lots of things make that bar and are silent. Fog, for example, or shadow. Creatures can be done and seen at a distance and not heard. They have more options for believability, and thus less likely to be investigated.
And, truthfully, there are very very few monsters that have good investigation skills.
I think the best use of Minor Illusion is to pop up a rock in front of yourself and shoot through it. If you are in a natural environment, the enemy shouldn't automatically figure out that the rock is an illusion since it's very possible for a rock to actually be there. And, to that matter, why do the bad guys automatically figure out it's an illusion? This is a Wizard they are fighting against; he can surely summon actual rocks, as far as the enemy is concerned. They need to *always* have that investigation or go physically put their hands on it to determine it's an illusion, otherwise, you are ruining the utility of the spell. Remember: the Wizard spent an action to cast that cantrip, which he could have used to cast Firebolt. So, I think it's fair that the enemy has to spend an action to determine it's not real (or they have to go physically grab the wizard through the illusion, which is another can of worms altogether!)