Even at the same timeframe things were different, like the transporter effects for the Defiant, Voyager and Enterprise E, even though they were all contemporary ships. That's leaving aside that the Enterprise in TMP is meant to be the same ship we saw in TOS and TAS, and even though it has undergone a refit spaces like engineering are a totally different size and shape. Or the pre-Enterprise explanation for the TMP Klingons looking different was "we'd always have done them this way if we had the money and the technology"
Like, I get how people want there to be visual continuity, but for the most part Trek is not the place to find it, even when they had concurrent productions. And yeah, DISCO is more of a radical shift than Trek has done previously, but the prior look and feel wouldn't have worked on TV these days - hell, a big part of why Generations was lit so darkly was to hide how awful the TNG Enterprise sets looked on film. Ultra HD streaming means that there needs to be an insane level of detail to feel 'real', which isn't necessarily compatible with the look of the TOS or the TNG era.
So, if you need to do a ground-up redesign of the setting, why pick the TOS era to set your new show? Well, the creators say they had a good story to tell in that era, so that's reason one. But also, it helps with recognition outside the die-hard fans - and the lesson of the last 15 years is that Trek can't survive on just the die-hard fans. It sucks if you can't get past the visual differences, but a decision was made early on in the show's production to effectively not cater to you.
