Unless the savings would be more than 100k Eur / 300k++ USD (I.e. total cost of one employee, it's not really worth it. Even then, moving to new infrastructure carries high risk for business disruptions which can cause an even bigger dip in revenue.

The cloud providers have definitely optimized their pricing for maximum profit extraction. Costs are high, and in many cases it's not high enough to actually warrant changing infrstructure to cheaper alternatives.

Sticking with AWS / Azure / GCP carries other benefits, too. You're more likely to find engineers who are experienced with those cloud platforms over, say, OVH.