Why aren't decoupling caps built into the IC or IC package?
Integrating capacitors on a chip is expensive (they need a lot of space) and not very efficient (you're limited to extremely small capacitors).
The packaging doesn't offer the room neither, the capacitor would be in the way of the bonding.

edit
IC package miniaturization is driven by the cellphone market (hundreds of megadevices a year, if not a gigadevice). We always want smaller packages, both in area and in height. Just open your cellphone to see what the problem is. (My phone is 1 cm thin, which includes housing top and bottom, a display, a 5 mm thick battery, and between that there's a PCB with components.) You can find BGA packages less than a mm high (this SRAM package is 0.55 mm(!)). That's less than the height of a 0402 100 nF decoupling capacitor.
Also typical of SRAM is that package size isn't standard. You find 8 mm * 6 mm, but also 9 mm * 6 mm. That's because the package fits the die as closely as possible. Just the die plus on each side a fraction of a mm for the bonding. (BTW, BGA dies are bonded on an integrated PCB, which routes the signals from the edges to the ball grid.)
This is an extreme example, but other packages like TQFP don't leave much more room.  

It's also much cheaper to pick and place a capacitor on the PCB; you're doing this anyway for the other components.