Who says? The EU. There are other statuses that exist like EFTA and so on that don't have this issue, there's no reason at all why this couldn't be agreed (except that the EU haven't the incentive to make this work, quite the opposite).
"Third country" is a term of EU law in this case, and refers to one country's status with regards to EU law - including how entities in EU can deal with them.
Indeed. Creating imagined barriers where nobody (eu companies/citizens, uk companies/citizens, or the uk government) actually wants or needs them. In practice IMO they are just there because they're incentivised to make things difficult for us.
