Same reason I want the US off the podium: it might be crap at the start, but depending on a single benefactor is never a good sign. I want to be able to say "well, I don't like it! F*** you!" and look for someone else.
the UK was able to say "I don't like it! F*** you!" within the EU with their veto right though
there is an argument to be made, that they might have less say over what they do outside the EU, as the future FTA will certainly necessitate that the UK implements standards the EU has agreed upon (now without them having a veto right)