So many reasons.
If the following conditions are met:
- opponent has units that are low to medium weight
- those units are outside of range of statics that could intercept phoenix
- opponents don't have air superiority
- those units are clumped
It's really good.
As are thunderbird, wyvern, rapier, banshee, blastwing, blackdawn, valk-roaches, and even raven, all of which are more resilient to surpises.
You're taking a greater risk (low HP/cost ratio) to deliver marginally better results within very specific contexts. Yes there are the dream shots where an opponent is all clustered, but that's a mistake you can't rely on an opponent making. Also, spreading damage is not as good as it might seem - doing a bunch of damage that doesn't secure kills costs your opponent energy and BP to repair, as well as tempo, but not metal. Other units are better at actually securing kills, which is very important.
Also, when you go for a bombing run and are shot down, you're typically moving in a direction from your factory towards their units, which often means that if you die, the wreckage is pre-secured by them.
I think there's a place for phoenix at a particular stage of the game where you've rushed planes as a second factory in a 1v1 - but that window is narrow when compared to the long utility life of other air units, and while it
might be competitive, most people aren't at a skill level where they can accommodate such technicalities for marginal gains. It's not useless, it's just not good IMO.
Compare two phoenix (720m) to three banshees (660m). The banshees repair themselves, are more durable, more granular, have amazing up-time, are very maneuverable, and wipe out all raider, skirm, and assaults outside your opponents air control, while pressuring to severely infringe on your opponents territory if they miss a beat. The phoenix can sometimes land big hits - but if opponents are playing competently they won't, they are fast but not maneuverable, have terrible up-time, have poor health, are less granular, require a skill shot, and have next to zero potential to threaten your opponents territory. It's not even close.