Because packages put stuff in specific directories. If you had a WordPress package, it would install to something like /var/www/wordpress. But lo and behold, you host two sites, so you put them in /var/www/site1.com and /var/www/site2.com. Or maybe in home directories. Or wherever. So now the packages are useless.
The Linux software packaging model is a joke for anything other than one-off appliances and hardcore nerds who have no life and want to babysit their computers. For all the headaches the Windows software installation CAN cause, out in the real world with real users, it causes very few headaches. It just works. And since the installers there let you pick any installation directory (usually) it's easier to have multiple copies of a piece of software and still have it tracked by the Windows "packaging" service.
The only thing the Windows model lacks that the Linux model has is a unified software update facility, but that is very much an easy to solve problem if anybody actually bothered to try. Instead, though, the Linux people stick with their pre-packaged pre-configured pre-mandated per-distro per-version fixed-everything packaging model. And real users suffer when they use Linux while the nerds go on and on about how easy it is to install and manage software... so long as you only need the software that was packaged for your distro, don't need newer versions of software which are only packaged for the latest version of your distro, don't mind updating every last component of your OS every 6 months and getting a ton of all new bugs and UI changes just in order to get the one or two bugs you needed fixed, and don't mind having insanely difficult installation procedures for inherently unpackagable software like commercial games that have gigabytes of data to install to disk.
But hey, this is 10th year that it's the Year of the Linux Desktop, so clearly users are all going to convert over finally despite Linux developers still refusing to solve one of the fundamental problems real users have with Linux.
(And yes, this is a real user problem. I used to be one of those "get everyone I know to use Linux" people. Then I found out that real people want to do things like install games and such without having to open up a fucking terminal and figuring out arcane magic commands to install it, and then do it over again with every game patch, when they could have been using all that time just playing the damn game, playing a sport outside, getting laid, etc.)