Multi-cloud is far from trivial, which is why most companies... don't.

Even if you are multi-cloud, you will be egressing data from one platform to another and racking up large bills (imagine putting CloudFront in front of a GCS endpoint lmao), you are incentivized to stick on a single platform. I don't blame anyone for being single-cloud with the barriers they put up, and how difficult maintaining your own infrastructure is.

Once you get large enough to afford tape libraries then yeah having your own offsite for large backups makes a lot of sense, but otherwise the convenience and reliability (when AWS isn't nuking your account) of managed storage is hard to beat — cold HDDs are not great, and m-disc is pricey.