Why don't they post min. and max requierments for games?

his should probably be in the Site Enhancements & Feedback section.
Some games do list the minimum requirements others do not. I agree a more detail would be nice.

If you look on the right hand side of the summary page for games next to the ESRB rating is a link that says Game Details. If the requirements are listed that is where they will be.

Here is a link to the ones for Star Trek Legacy for example LINK

~edit
I would like to add that those minimum specs the link takes you to suck.
This is what should be there.

This is from the box

MINIMUM SYSTEM REQUREMENTS
Windows XP (Home, Media Center Or pro Editions) with Service Pack 2; DirectX Version 9.0c; Intel Pentium 4 (2.66 ghz or higher) or AMD Athlon XP2800+ or higher; DX9 compatible AGP 8X or PCI Express video card, 128 MB VRAM, Pixel Shader 1.4; 16-bit Direct Sound compatible sound card, speakers or headphones required for audio; 512 MB RAM; 5 GB free space; LAN or Broadband connection (DSL or Cable Modem) for Multiplayer; Windows compatible Keyboard and Mouse.

Video cards supported (not on the box)
ATI X1900 series
ATI X1800 series
ATI X1600 series
ATI X1300 series
ATI X850 series
ATI x800 series
ATI x700 series
ATI x600 series
ATI Radeon 9800 series
ATI Radeon 9700 series
ATI Radeon 9600 series
ATI Radeon 9500 series
NVIDIA GeForce 7800 series
NVIDIA GeForce 6800 series
NVIDIA GeForce 6600 series
NVIDIA GeForce 6200 series
NVIDIA GeForce FX series (5700 or above)

I'm not a pc gamer but every once and a while I will play. Some sites will analyze your pc against the game and give you a point by point breakdown of how well your pc will run. Something like this.

http://www.nzone.com/object/nzone_twimtbp_gameslist.html

Click on a game and its scans your pc and gives you a good idea. There' a few sites that do the same thing.