This is true only if your time is free or you're cutting corners on reliability. For example, using an EC2 medium instance on-demand is $0.120/hour and RDS is $0.160/hour. If you use enough MySQL, that $0.04/hour (in reality, $0.022 at reserved rates) can reach the point where it's worth rolling your own management stack, backups, failover, etc. but there are many people who will never reach the point where they don't have higher-priority investments to make.