Pearl wasn’t there to hear Jasper call fusion “a cheap tactic to make weak gems stronger”. She didn’t see the forced cluster fusion. So even though she knows what she did was wrong I don’t think she really understands how much it hurt Garnet personally.
The last thing Garnet said before she stopped talking to Pearl was “Peridot is out there somewhere and Pearl’s been distracting us with nothing!” so Pearl thinks if she finds Peridot everything will be okay again, because she thinks the only thing Garnet’s upset about is that they wasted time.
Garnet has every right to be mad but I don’t think she fully understands Pearl’s motivations either. She has a lot of strength and confidence that Pearl doesn’t have so she might not see why Pearl would go through such lengths to form Sardonyx.
Pearl can’t personally identify with being a fusion all the time and Garnet can’t personally identify with how lonely Pearl feels because she’s “never alone”.
So yeah, they really need to talk.