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)