I don't see it as massively different to specialisation between front and back-end developers though - we're all devs, we're all comfortable with reading/writin

I don't see it as massively different to specialisation between front and back-end developers though - we're all devs, we're all comfortable with reading/writing code, but whereas my specialty and focus is on application-level (typically backend) code, the DevOps guys are specialised in writing IaC scripts etc. Yes, in principle, any dev could do it all, but there becomes a point that the mental load from juggling too many technologies outweighs your ability to be productive. And some types of development just take different mindsets - I've done my share of front-end/UI-type development in the past, and can easily pick it up again if needed, but I can't say I find it super satisfying, and it's nearly always going to be produce a better result for everyone to hand it off to someone who does. Likewise for CI/CD scripting. And it also happens that if your primary focus is on the scripts and tools necessary to get software deployed to a particular hosting environment, you're likely to have the skills necessary to take care of the day-to-day sysadmin side of things for that environment (indeed, most "manual" changes to the environment are only done as a last resort, and would be wiped by the next deployment anyway, unless the scripts are modified appropriately too).