Because it’s not their “anthem”. They think You’ll Never Walk Alone is special, they think it means more than other “anthems”. They think Liverpool’s traditions means more than other traditions.
Abide With Me is not YAWN so they belittle and disrespect it. They think YAWN has a bigger tradition and history than AWM and think when other sets of fans sing and boo over YAWN, that it’s an affront to their club, so they despise anything that’s not YAWN.
Like you say, they’re fucking weirdos. They don’t see any of the irony or hypocrisy that seeps out of them.
They boo God Save The Queen because they say they’re “Scouse, not English” even though GSTQ is the national anthem of Great Britain and Northern Ireland, not England (England doesn’t have a national anthem). They think that the govt has always hated Liverpool even though all British port and pit towns and cities have all been treated the same as Liverpool. They think there’s a national hatred of everything Scouse yet completely miss the fact that 90% of their entire fanbase aren’t Scouse anyway (that we and other fans call Liverpool fans “Scousers” is an insult to Everton fans!).
They think they’re more important than everyone and everything else so they get to do what they want. Just like United fans (who used to chant it to tell us all; “we’re Man United, we do what we want”!). Two cheeks of the same arse.