Phone numbers actually can be an identity, but generally that's just business numbers. There's also a lot of phone numbers that aren't cell phones and can't get texts. It's a lot easier and cheaper for companies to design one system. Depending on the company, they'll still do text/email reminders for things. Old people like phone calls, so companies need to know their audience too. Again, cheaper to build a single system, so that tends to be the default.
Source: I design call centers for a living, personally done probably 200+ projects over the years, in banking, insurance, retail, transportation, state/local govt, etc.