Cost. You're buying the cheap ones!
Paracetamol is a very commoditised API and cost is king - by reducing the excipients to not a lot more than talc or sodium carbonate + a few other bits for compressibility and dispersion, you can get most of the cost out.
Plus certain grades of Paracetamol compress into tablets really easily so you don't need much else.
Ibuprofen is a bit harder to get into a tablet and I think the tabletting process works better with Lactose which gives a sweeter flavour.
My job is very dull at times but hey ho, every so often I question pops up that I unfortunately know the answer to...as you can imagine I'm a hit at parties.