Because they were showing up as actual armies as part of a campaign that was fighting on few enough fronts for them to actually be able to show up a lot of the time?

A bunch of separate forces, stationed in different places, etc. are going to be able to participate in a wider range of conflicts.
If you are making a claim that there are fewer conflicts in 40k than there are in pre-Heresy era, I'm going to ask for a source.

No

In which case it seems very much like what you said was just your headcanon that is not supported by the official material.

Even if Space Marine fanboys have tried making them less silly in more recent editions they're still an absolute shitpost of a concept

I.e. they fit much better with the pre-3rd edition version of the setting? I agree. And my point has been from the start that they make no sense in a setting that takes itself seriously.

But all that is kind of getting away from my original point, which was that the Imperium itself does "just make tanks instead,"

But it doesn't. In both the pre-Heresy and 40k eras they do make more SM, and even develop the Primaris version instead of just using the resources for more actually useful stuff.

Space Marines only get made by the tiny independent Space Marine chapter-states that are allied with but entirely independent of the Imperium

Their relation to the wider Imperium is closer to being pseudo-feudal vassals of the Imperial government. I wouldn't call them 'entirely independent'.
OTOH, this is not particularly relevant to the topic, and is more of a nitpick.