Let's be honest, based on his current form he does not deserve to be starting for England. Reece James is in better form at the moment. Trent hasn't looked right this last couple of months and the effort hasn't been there. If Southgate really was a manager who picked based on form, this would be a sensible decision. The thing about Southgate is that he isn't, and Trent will not be a starter for England until the England manager changes.
Having said that, over the last 4 years he's been the best full-back in Europe, and no England full-back has come close to what he's done. His attacking output speaks for itself and he's one of the most creative players England have at their disposal, but let's not forget that despite being 'terrible' at defending he's been a mainstay of a defence that has conceded the fewest goals in 3 out of the 4 previous league seasons. He's not got the credit he deserves from what he's done in this time.
The fact that he's never been fancied by Southgate has been a constant stick used to beat him, and I'm sure that there wouldn't be so much talk of his defensive flaws if this wasn't the case. Actually, I don't remember much criticism about his defending until just after the World Cup in Russia where it became clear Southgate preferred Tripper/Walker over him.
For now England have a manager who is taking a bunch of decent players from high-energy, pressing, attacking teams (City, Arsenal, Liverpool, now United) and has set them up in a rigid defensive table-football system. The situation with Trent highlights his limitations as a manager, rather than Trent's as a player. I'm sure the next manager (who sees the potential this England team has in an attacking system) will find a way to fit one of their most talented players into the side.
On a side note, I saw a picture of him in England training next to Henderson and I didn't realise how much bulk he's gained this summer. It doesn't seem necessary.