Well, then you don't agree to it.

> I cannot modify the software? Why not?

To spare the company supporting randomly modified versions.

Also, the "cannot modify" clause is a weaker version of a universally used "You shall not decompile or reverse-engineerthe software", which is also frequently coupled with "You will not try and defeat, deactivate, bypass, remove or otherwise circumvent any software protection mechanisms". I don't think it makes much practical sense, but this is an industry standard. Virtually every proprietary software license is locked down this way.