Why new Linux installation does not treat already installed Linux at the same way as Windows. Why do you have to struggle to boot several Linux distros from one machine? Dual boot of Windows and Linux is taken care during installation process. Do you think that the fierce competition between Linux distributions is the reason for this? Do you think it should be as easy as with windows ?

This is one of those questions that often does not have a rational answer...

Some similar questions:
Why does not MS offer multiboot options when installing?
A: To them, there is no world beyond Windows (Kind of like the movie Pleasantville)

Why do typical Linux distros auto-detect and set up for dual-boot with Windows?
A: Because that is a very common scenario

Why not auto-detect other Linuces?
A: Because that is NOT a common scenario.

I think that most distro authors assume that--once you start installing multiple versions of Linux--you know how to set up you own bootloading config.

My expereince has been that a second Linux installation recognizes the first one as easily as it recognizes Windows. When a third installation is done, things start to get a little trickier, whether or not one of them is Windows.

My Ubuntu recognised my Slackware OK.