That's actually not even necessarily true, the Helm Datacore actually had a lot of general everday stuff, more so than military tech.... And also based on the previous examples I would put it to you that the aesthetics of the inner sphere are different to your prefences

One of the great things about Btech has been its inconsistencies. These are meant to be diverse nations, with diverse cultures and aesthetic values. You may think Simple is the best, but in a time where a fancy decal may be easy to add to a mech(Magic Paint sprayer tech

), other cultures may look at the more complex imagery as the ideal. There is no reason in the fluff not to have lots of differences.
And Yes I get that the inconsistencies came because FASA borrowed from all over the place. But the one thing that did was give a feeling of diverse nations and cultures. Look at the different Mech. They look so different sometimes and yet none of them feel "wrong".
It's not a question of "fancy decal(s) may be easy to add", it's a question of what is visually cohesive and distinctive at a variety of distances. Insignias with lots of small detail look like a messy and indistinct blur at a distance, which is a big problem, because you
don't want to be misidentifying friends and foes on the battlefield.
Let's face it, a lot of the old insignia art is kinda janky. Not like it was/is exceptional in that regard, compared to art for other PnP games, but let's not pretend that it's good because FASA put it together. That doesn't mean people aren't allowed to like it, but we shouldn't deny the technical flaws. This goes for all kinds of media. For example,
The Beast of Yucca Flats is B-movie trash, and even though I love it (in part
because it's B-movie trash) I'll be the first to admit it's technically flawed in practically every conceivable way.
We can appreciate the diversity of FASA's insignia art while acknowledging that a lot of it is technically flawed, and, were HBS to emulate that style when making their own insignias, they wouldn't render well in game. Additionally, a good many would end up being pigeon-holed and unusable for the kinds of characters and merc companies that people want to make, because an insignia with lots of "character" is not generalizable. I make a new custom insignia for each character/merc company I make for precisely that reason; North Star Sultana and 21 ACES do not have insignias that another merc company could pick up and use without a problem.
It's my guess that: HBS went with soft colour palettes because we didn't initially have bright colours to choose for our merc companies, look at the non-bright colours and the stock insignias mesh with them quite well; they went with simple, distinctive designs so that you could easily pick them out when they appear on the UI and, more specifically, on your 'Mech if you've got alternate skins; they included a lot of symmetrical and geometrically pleasing designs because those tend to be universally eye-catching and attractive, no matter what culture you come from.
But, knowing that a good portion of the player base would never be satisfied with stock insignias, HBS made it a very simple matter for us to add and use our own. They built for a larger audience, but left the game files open so that the fewer who wanted something different could get something different with little effort. That's what I call "the best of both worlds".