Well for one, this is Halo 1, not H2V, there are different networking models at work. Wire-frame isn't really that advantageous, I'm not sure what you mean by blue walls or "chams" or what blue arrow does, and the only real "trainer" that can really make a difference is an aimbot which we have ways to detect (both automated and via human auditing).

In a PC environment there will always be cheaters, its just a matter of fact and way of life. Even if a client sided application was made, one can easily tamper with such a system. Security through obscurity only lasts for so long so its just one of those times where its just better to be defense (with various documented detection, transparent to clients) rather than offense (external application causing more network activity and CPU useage on both the client and server)