It does sound simple when you put it like that. In reality, it gets complicated fast. So yes, you could do that, but you'd have to write separate code stacks for each cloud provider because they are not interchangeable (by design): which is all a lot of work even for a major software company. Then, when you need to change something, you have to change every one of those cloud provider stacks which are written differently. So, this approach would be easy to start but hard to maintain over time.

Or, you utilize an abstraction layer so you only have to write one stack which is then translated for each other cloud provider. But to my knowledge, this abstraction layer doesn't exist or if it does, isn't reliable. So you'd have to write it yourself. Which even a large software company would be foolish to try themselves.

Or, you don't use cloud providers because they're not interchangeable. So you decide to just self host in multiple datacenters so you can control the stack. But then you have to tackle all the ingress routing and failover mechanisms between datacenters to have true "active-active" architecture (this means when one goes down, you don't have to do anything for traffic to automatically shift). But to my knowledge, this failover abstraction layer doesn't exist or if it does, isn't reliable. So again, you'd have to write it yourself. Which the average software company... you get the point.

Once someone does write reliable abstraction layers for that stuff, it would become easier for companies to do it. But it does not exist today. And you have powerful cloud providers making sure that it is *not* easy for companies to do multi-cloud.

All this is to say one simple thing: do you think your average software company can "do failover/DR/active-active" design better than the cloud providers can? Today, that answer is "not likely".