why we couldn't see those games on wii. since last gen consoles have had gta, mgs and shooters then why more powerful console(wii) could not have them? it's all up to developers.

It could the cellz will make it happen.

Easily. The Wii uses Dvd's, same as the 360 so no troubles there
If you downgrade graphics then the slower processor is not a problem.
The size of the area that can be loaded at once might be the only consideration due to RAM, but just by reducing the complexity of the textures it can be done

one more thing to add to this comment, MGS4 had uncompressed 7.1 sound remember.  Go back to 5.1 (which I think the ps2 was doing), 480p instead of 720p and MSG4 could have had an iteration on the wii, though there would be a lot of changes (switching weapons and items would be....interesting).

with the eyetoy and a balanceboard you could probably make something similar to wiifit

Games can be ported down but it's not the same experience. I think that's fairly obvious. The Wii is about as powerful as an Xbox, which was more powerful than PS2, which had these types of games. The experiences have improved, though.

Do all Wii owners imagine that they purchased a console that will, someday, be able to play all the HD console games, merely with a minor loss in texture quality? Textures take a fair chunk of memory, yes, but... its hardly the sole contributor to the memory footprint of an application like GTA4 or AC. Lemme throw up a sample memory budget for a run-of-the-mill HD game:

* 30-60 MB -- OS (I think the Wii has a much smaller OS)
* 4 MB -- screen buffers (easy to shrink if you lose HD)
* 60 MB -- code and global data (next to impossible to shrink without changing the game)
* 30 MB -- game objects (next to impossible to shrink -- see above)
* 20 MB -- animations (easy to shrink... but with substantial quality loss)
* 60 MB -- physics/collision data (pretty hard to shrink without serious level changes, which means you're re-doing all the content)
* 120 MB -- textures (easy to shrink, acceptable quality loss, IMO)
* 60 MB -- visual geometry (kinda hard to shrink, but somewhat doable, depending on artist skills and tools... and time)
* 20 MB -- sound (easy to shrink if you lower quality and compress like crazy... maybe)
* 20 MB -- AI navigational data (pretty dang hard to shrink without really messing up the AI and re-doing the content)
* ~60 MB -- misc crap I forgot (most stuff is usually much harder to shrink than textures)

Now fit that on the 64+27MB of the Wii, after accounting for, I would hope, 8MB or less for the Wii's OS.  You come out with a completely different game, that is also lame compared to its HD counterpart(s).

And yes, HD cross-platform games ALWAYS use ALL the memory available. I don't think I've ever heard of a HD game that didn't have to squeeze into its platform(s) memory budget at some point in its development cycle. This has been true for... eons. Consoles never have as much memory as the developers want.

I would love to see those games on Wii.