Politics does play a big part in this. Why else would they have those 'stupid rechecking rules'? I'm sure the airlines would rather avoid the hassle. Having to pass immigration even when transiting is also the governments fault.

As for LHR that'd still be a nightmare to transit even if the UK had no immigration controls due to the design of the airport. This should improve when T5 is complete where the plan is to move airlines around so each alliance is in the same terminal whether possible.

Leaving the UK is the worst part though as our carry on rules are more restrictive than the US.