If planes can go faster, why don't airlines fly faster?
The key issue that determines the aircraft speed is economics. Airlines fly the aircraft that make most sense economically- which is rarely, if ever, the fastest speed possible (more speed equals more drag usually, which reduces fuel economy). More fuel efficient the aircraft is, better the range/passenger capacity, making airlines more money.

The top speed of the (subsonic) aircraft has changed only a little in the past few decades- most developments have been geared up towards improving fuel efficiency, not increasing speed (speed alone doesn't make economic sense to airlines; look at what happened to Concorde). 

There is another issue here- most of the passenger aircraft fly in high subsonic (transonic) regime. As a result, the airliners are flown below the drag divergence mach number, beyond which the drag increases rapidly. This limits the aircraft speed.

As a related point, due to the high altitudes at which the aircraft are flown is that there is little speed range to chose from between the stall speed (lowest speed possible) and critical mach number (highest speed possible beyond which flow separates due to local supersonic flow). This limits the practical options available to the pilot.