I wonder why Eurostar was set up like this instead of treating it as a normal train that has a UK immigration stop near the UK tunnel entrance and French immigration at the French tunnel exit. Yes it would mean all trains make two stops but it would allow direct no transfer trains from anywhere in the UK to anywhere in Europe all for a couple of hours of inconvenience at the tunnel. Eg you could take a train from Manchester to Rome and only get off just for the immigration stops.
The simple answer is that those controls take a lot of time. It's much, much easier to have the controls on departure, as you also don't have to deal with train security issues.
It's actually not a problem in the UK->mainland Europe direction, as entry controls can be set up almost anywhere. Schengen countries are well set up for mobile border controls, and so you could easily have a border crossing at any station you choose. The issue is in the mainland Europe->UK direction, as the UK barely has any culture of mobile border controls, combined with an inability to return passengers to France if they're denied entry into the UK.
There are emerging solutions in the field of border control that could allow for remote UK border controls to be carried out. For example, in Estonia, there's at least one border crossing with Russia that became fully automated in Setomaa. In future, combined with the UK ETA, it's conceivable that UK entry controls could be placed at stations like Marne-la-Vallee and supervised by French police. These would be fully automated and only accessible to travellers able to use e-gates, with other passengers directed to Gare du Nord at the time of booking.
Isn't that what Ashford International and Calais Frethun were for? Both closed by Eurostar without any public consultation whatsoever.
Yes, very much so. Calais Frethun never made that much sense however, except in the context of local politics. There are a few TGV onwards connections a day to Paris, but that's about it. If there were more connections onwards, perhaps there would be some demand for a regional Eurostar service that would have both French entry/exit controls in Calais as well as UK entry controls there.