TLDR - Because no gas. Best option is to keep any of your mutas alive and keep them inside main/third base as dropship defense and possibly using them for what you said later
Long - Nerfs to zerg, everything costs too much gas, have to spend even more gas into scourge unless you want to lose to 2 dropships, all the upgrades/hive/defilers adds up. People used to say 4gas is when Zerg starts to spiral out of control but they run out of gas from main/natural eventually and have to expand again. If Terran keeps their vessels alive and just keeps irradiating then zerg can't win. It's also just the fact having to micro mutas and keeping them where they need to be takes too much AMP, zerg has to micro too much. In terms of costs Terran has perfect mineral/gas expenses, for zerg and protoss it's much more lopsided which is what makes them weaker/prone to dying to stuff