Well, because for a short while, it was assumed that UK would just follow the same rules as EU does, and thefore UK companies could be given an easy pass operating in the EU, or when interacting with EU citizens. For instance there is/was no need to inspect any items coming from UK into the EU, since the general assumption is that the standards are still the same as they were pre-brexit, so they don't need to be treated like coming from a country like China or India.

However, this is all changing now. UK is keen to get rid of regulations it inherited from the EU, making it a much less attractive place to do business with, from the EU perspective. We can no longer assume that companies hosting EU citizen's data on UK soil will play nice and follow the same rules - it will be treated with caution initially, and (hopefully) with eventual hostility towards it, as UK seems to be going full on surveilence mode with government and its agencies having access to everything at all times with no warrants needed.

So yes, it "doesn't matter" only if UK isn't interested in working with the largest trading block on the planet. If they truly want to go that path, then sure, whatever.