Sure, you could purchase your own drives. But then it wouldn’t be offsite. And it probably wouldn’t be on a redundant internet connection. And most likely not have redundant power and cooling. And, and, and.
Self-hosting at home (or in the office) is a great option for some if you’re not worried about needing an offsite backup. For those that do care about this sort of thing, though, the extra you pay to have someone else manage the thing is well worth it.
It can be off-site if you have a friend with Internet where you can stash that disk attached to a raspberry pi. Quite a few people do that and it means you're storing terabytes not at the steep $300/year but at $30/year for every terabyte. I can eat healthily for a month from that difference, that's worth a few minutes of effort setting up port forwarding and installing zfs if that's your thing (personally I'd leave it plain ext4 and dump something encrypted like Restic on it, taking all of 5 minutes to setup from downloading the OS to Pi Reporting for Duty).
Calculation: pi 40eur, 1TB external disk 50eur, typical lifespan of a disk 5 years (when excluding including infant mortality which falls under the mandatory 2-year warranty for new electronics), ~8W power draw is ~€15/year. Let's say you also need to replace the pi after 5 years just for good measure. That's 15+(90/5)=€33/year for 1TB, which gets cheaper per terabyte with bigger or multiple drives.
Self-hosting at home (or in the office) is a great option for some if you’re not worried about needing an offsite backup. For those that do care about this sort of thing, though, the extra you pay to have someone else manage the thing is well worth it.