installing a new stereo in my 99 camry. why cant they make a wire harness, that just snaps in BOTH sides?

Because most adapters are generic. They cannot possibly manufacture every combination there is without losing a lot of money on items they may only sell a few of. You've got an 18 year old car for Christ's sake. Get real!

Because Sony uses a different plug than Pioneer, who uses a different plug than Alpine, who uses a different plug than Kenwood, who uses a different plug this year than they did last year...

You won't find many adapters that connect directly from your car to an after-market head unit, because different cars have different plugs and they keep changing, and different head unit manufacturers use different plugs and they keep changing too. Even different models from the same manufacturer might use different plugs. So the best option is the one you hate right now: buy the right harness to fit your vehicle, spend five minutes connecting it to the plug that came with the new head unit, connect it up and enjoy.

The schematic for the car is simple unless you're color-blind.