I think that's part of it but it's also partly the legacy built environment. California leaned harder into auto-centric infrastructure than maybe anywhere else in the entire world. That's starting to change but rebuilding an entire transportation system takes time.
Especially when public funds are scarce. Prop 13 does have a part here since tax increases generally require a supermajority of voter approval to be implemented.
Even here in Sacramento (we're #20 on this list I believe) improvements have been slow despite there not being a single republican leader on the city council in recent memory. But they still won't put real funding into solving these things, instead focusing on symbolic improvements that don't address the problem.