I've heard this before but, as a Scouser who's been four times in a row (with my fifth upcoming 🥳) I think this is just copium from people who fail to get tickets.
The reason you see (or you think you see) loads of Scousers round the site is a combination of:
Glastonbury having semi-mythic status amongst Scousers. I was always told this was because the security firms all came from Liverpool in the 00s (presumably ex-gangland/door staff?) and spread the word about how great it was back home. Simply put, it just seems to mean more to people in Liverpool, and we're a bunch who like to follow the trends of others.
Scousers stand out. The accent plus the uniform mixing sportswear with hippy chic makes us clearly identifiable. I'm sure there's also thousands of Londoners, for instance, but they're less sartorially homogeneous, so stand out less. Plus, being the trendsetters we are, it's caught on outside of Liverpool too - so a good chunk of the people you visually clock as Scousers may well not be, but it makes you think there's more of us than there are.
Possibly linked to point 1, I think Scousers (especially the young ones) have fewer qualms about getting in via unorthodox methods. Liverpool is a little village, and the odds of you knowing someone who knows someone who can get you in the back of a van are pretty high.
Linked to point 3, a lot of Scousers are quite happy to put down hundreds, if not thousands to get hold of a VIP ticket or to buy a ticket off someone who looks like a bit like them. Our flash-the-cash culture, being (unfortunately!) the credit capital of Britain and the legion of young Scouse men who seem to carry wads of cash for reasons I couldn't possibly speculate on all engender this.
So I'd say all of the above in combination are probably the reason you see more Scousers on site rather than a sinister cabal hoarding all the tickets.