Question is pretty much given away in the title...
I noticed that the liquid and gaseous lines are often separately insulated with foam, and yet from what I understand about refrigeration, I'd expect to have efficiency gains if the 2 lines are in good thermal contact with each other.

More context:
I am speaking of common home-use air-conditioners, in tropical climates (30C (86F), 85%RH) where there is a indoor evaporator-fan-coil-unit linked with an outdoor condenser-compressor-fan-unit, via 2 copper pipes.

I expect the temperature of the liquid line to be slightly warmer than outdoor-air, and the gaseous line to be colder than indoor-air, so if we let the 2 pipes be in thermal contact, (for starters, don't insulate them separately, do put them in the same foam tube), wouldn't we get more efficiency for free?

What am I missing? Why are those lines normally insulated separately? (Yes, I'm a lay person...)

TIA