I guess because it is all self contained and does not need access to /Library for some reason.

If you wanted to install it from the command line you would have to download the binary and compile it yourself, or use the mount and cp or mv commands to move it form the disk image to the /Applications folder.

If it was self contained though you can run it from anywhere, not just the /Applications folder.

If I recall, by default OS X 10.5 has PHP 5, MySQL and Apache installed by default. The only one I am iffy on is MySQL, not sure if it is installed by default.

So really you just need to edit your Apache config file to turn it on and point it to your web root folder which should be ~/sites or something like that (memory is drawing a blank atm and my macbook is in my laptop bag and I am being lazy....sorry)