Why inductors can not be integrated into IC?
They certainly can be.  The problem is that they're not very well behaved.  They are only used where absolutely necessary.  For low frequencies, they're pretty much out of the question as they would be too large.  For RF, they are used regularly to build filters and baluns and what not.  However, inductors built with planar processes have lots of nasty parasitics that prevent them from being very effective.  Things like parasitic capacitance can cause inductors built on ICs to have low quality factors - meaning they don't really behave like an ideal inductor, but like an RLC circuit that's 'mostly' inductor.  Also, inductors are more problematic when it comes to external interference - they are more likely to cause it as well as more likely to be affected by it.  It is also likely for inductors on a chip to cross-couple to each other, which is very difficult to avoid due to the physical constraints of a chip - physical separation is required to reduce magnetic coupling, but chip area is expensive. 

If you want to see pictures of inductors on chips, do a google image search for RFIC or MMIC.