I have switched carriers several times in the last 15 years, usually for data or pricing reasons.

I'm not atypical. It's easy to switch in the UK, easy to port your number, and people have little carrier loyalty. Switching home broadband is more of a pain, but switching cell carriers (or, rather, "mobile providers") takes a phone call and a day of questionable connectivity and that's it.

If people are paying for data on one carrier, that another carrier is offering for free (for whatever reason), that can be a reason to switch; that's why T-Mobile is offering it. In the US, it's relatively simple to switch your cell phone service, and there are many choices: 4 mainstream national carriers, all of them have prepaid brands, and many MVNO options as well.

For home internet, if you have a choice, it's usually between only two companies with different technology, and often significantly different bandwidth options, there's not much room to compare on price, and very little room to compare on routing or terms of service.

Anecdotal (counter)evidence: in India, Reliance ("Jio") has been offering free (which is a first) 4G (I believe only one other carrier offers that) data for a few months now, ending March of this year. (I believe they're using this to both draw users and give people a taste of what a fast mobile data connection can be like as well as to work out teething problems. Nobody's a saint, etc.)