I wouldn't even say the ramps, I'd want to allow passengers to actually be able to wait on the platform even if the train isn't ready yet. I've never seen Liverpool Lime Street close off the platform, they either just let people on, or keep the doors locked until it's ready. But in Euston it genuinely feels like a criminal offence to even hang around on the ramp. It feels more like an international airport than a public transport hub.
Platforms do get closed off at Lime St, presumably you do mean 6-10 as they aren't barriered, but only for large events. Usually managed by TOC revenue and BTP teams.
It's probably down to passenger numbers - for example;
LNWR Euston - Crewe regularly has 400-500 passengers boarding
LNWR Liverpool - New St regularly has 150-180 boarding.
Now granted, the trains are longer for Euston services than Liverpool, and would hopefully spread out down the platforms, the reality is both stations have big issues with crowding at the concourse end of platforms, making it difficult for those departing the train to get off and subsequently leave the platform.
This is something I see every weekend at Lime St for instance, and something that I'm glad I don't have to see at Euston (other than when platforms are announced ahead of the incoming train service which has happened before and is a very dangerous situation)