Keeping the same number is more convenient for the people that you do want calling you.

I imagine that for some, it also contributes to a sense of identity, much the same way that a mailing address might.

I imagine that in this modern world people have kept emails longer than phone numbers or physical mailing addresses

I'm one of those, as far as I can tell, I've had the same cellphone number since at least college, possibly high school.

Where do you live, and why do cell phone numbers cycle so quickly?

A mix of moving abroad a lot, but also change of phone provider (often moved provider because they were cheaper, and didn't manage to keep my number). I also never really made an effort to keep the same phone number as most people used email, then facebook, then other online apps (whatsapp, instagram, etc.) and I rarely used my phone number for anything long-term relationship

Because it doesn't matter. Or it didn't in the past, now it's a security nightmare to not have your number anymore and risk someone else having it before you changed it everywhere.

Now that lots of services require you to have a phone/use a phone number to login changing numbers could potentially mean losing access to services/accounts you use. (I also hate this)

Swapping my phone number every year in the US would be an annoying as hell. Tons of services use phone number as 2FA or a backup recovery. (Including a lot of banks) I use SMS with some people and that would cut contact with them. Same direction if they changed numbers.

using a phone number as 2FA is generally bad practice but yeah I see what you mean. For anything else I think using whatsapp makes more sense

Yeah, I've had the same number since about 2001. It's nice as I've moved since then so any number that calls from my area code is definitely spam, although that's not really an issue now that my phone doesn't ring for unknown numbers.

Changing phone numbers is like changing email. You risk losing access to some services/friends.

I haven't had any friend use my phone number to reach me out since like 2008?

I'm 38 and have the same number I had since 1999.