Balance. The infusions as they stand largely reproduce the effect of magic items the party might find anyway. So their actual effect of infusions on the artificers combat power is negligible*. However, the combat pets add significantly to the artificers combat power and do not reproduce the effect of something that could be found anyway. Ergo, artificers would be forced to take the pet infusion anyway or be gimped in combat. A choice between something powerful and something mediocre is not a choice at all.
*Repeating Shot in conjunction with a pistol or musket gives the artificer a significant boost to combat power, but that is situational and build-specific enough to be excused.
The suggestion that infusions don't really affect party combat capability because the party would find the magic items that they want anyway is a pretty massive assumption.
I would put a having homunculus as closer to having a couple of extra magic items in terms of concept and class design space than spell slots. I don't have a problem with homunculi being an effective way of spending infusion points, but I really don't think that its is that easy to "gimp" a 5e character.
Someone who likes the concept of the artificer crafting a companion can get one without it being required for someone who doesn't like the idea of artificer pets. Maybe they'd rather have a wand, or a flametongue sword or some other way of increasing their personal combat capabilities more directly than a pet attack.
But again, if we’re just reskinning spells, we don’t need a new class for that. I can already just play a wizard and pretend he’s making potions, bombs, and gadgets using a tool instead of a magic wand. To justify its addition to the game, the Artificer needs to do something that existing classes don’t already do, and “cast spells, but pretend they’re not spells” is absolutely something other classes can do.
The artificer already does do things that other classes can't. You seem fixated on the spells, when they're really not the entire focus of the class. Other classses retain identity despite the fact that they also have spell slots and can cast spells, and the artificer does as well. The "casting spells from items" schtick is a way of merging compatible mechanics with class concept but its not an artificers core identity. You could play a wizard like that - but you could also play a cleric, or a bard, or a ranger, or paladin etc like that as well, and they would still be distinct classes.
Infusions, capability with tools, companion constructs and such like are things that the artificer can do and no one other class can. Those are part of the class concept at least as much as the artificer's ability to mix up a healing salve or hotwire a lightning bolt from an amber necklace, a couple of runes and an angry rodent.
Yeah, what I want out of the Artificer is D&D Batman. A character with no super powersspells, who can still keep up with the superscasters by using various tools and gadgets.
That is indeed an interesting class concept, bit I don't think that it matches the Eberron Artificer that is being put together here. Remember the basis of the class is someone who understands the principles of magic possibly even better than a wizard does. The artificer isn't just someone who uses magical tools, they are someone who creates them.