In general, banks compete on other attributes - a small difference in a mortgage interest rate is a lot of money and makes up for a HUGE difference in the quality of internet services; and whenever loan market becomes tight (and thus it's not attractive to refinance to another bank) people are pretty much locked in.
Exactly this. They're not great, but there's something about being a customer somewhere for a very long time that genuinely does (or did) seem to offer you a mortgage rate that was advantageous.