Because it’s not ambiguous.

If I pay for something in Australia and the bill comes to $50 this has meaning within that context.

I receive a bill in Zimbabwe for $50 this also has meaning within that context.

These values are not equivalent.

Ditto if I were to say it’s 30 degrees out. You may interpret that as either a good day for the beach, nice weather for ice skating, or we need to bear north-northeast depending on what context we share.

Language is messy.

This is unnecessary complexity. My rant is against that exactly. You're defending confusing shit that doesn't get any better from being confusing. If all countries had a different currency, things would be clearer too. Ask any Australian shopping for digital products on international sites. Half of the sites write $ but forget to specify whether it's "we geolocated you and guessed AUD" or "haha it's USD but we just wrote $ because we forgot that there's a world outside the US". If Switzerland would rename the CHF to Euro but not change its value to match the existing Euro, everybody would agree that that's a terrible idea. There wouldn't be edgy HN commenters explaining that well, actually, there's precedent so it's fine! No, it'd just be bad. The dB ambiguity is a mess for the same reason. The situation has no benefit and in the world of units, where most other things (eg the most of the SI) are actually relatively usable and non-ambiguous, it's a fuckup. And the power vs voltage aspect of it makes it even worse than the $ situation.

Your argument that it isn't so bad in practice doesn't change the fact that it has no benefits whatsoever.

It's just ambiguity for the sake of it, because way back when people started measuring sound stuff, nobody bothered to go "but wait is this actually handy?" and then we got stuck with whatever the first guy came up with. It's just like the whole kilobyte/kibibyte crap and the whole Wh vs mAh vs kilojoule soup. It's all downside.