It's a single market on paper as the eu only mandates a small subset of common rules and regulations such as removing tarrifs or freedom of movement, but have you ever tried in practice to launch your company from Belgium to France or from Netherlands to Belgium or from Austria to Germany, or from Romania to Italy?

It's much more difficult when the rubber hits the road as every country has various extra laws and protectionist measures in place to protect it's domestic players from outsiders even if they came from within the EU. And that's besides the language barrier which means added costs. This is much less efficient than the US market.

EU countries and voters still value their national sovereignty and culture (both with the upsides and downsides) above a united EU under the same laws and language for everyone, ruled from outside their country's borders. See what happened with Brexit and the constant internal squabbling and sabotaging over critical EU issues that affect us all like the war in Ukraine or illegal mass migration. An US style unification just won't work here since every little country wants to be it's own king while having its cake and eating it too.