Never did, but they are faster than mainframes of just recent memory. Again, most companies do not and could not use such power. Reliability seems to be the dominate criteria right now for purchasers, and clustering gives them that. Distributed clustering is even better. My last large mainframe installation was a dozen 370 systems clustered across the globe. While an IPL was required of at least one system each day, we were off-line only 14 minutes the entire year, on average. That level of up-time is had also with clustering on Intel boxes at a price 1/10th that of a mainframe. While the mainframe might be faster, that level of power is not a norm.
Anything that needs terraflops of power will most likely be a parallel process and can be clustered. Even mainframes use multiple processors, they are just in the same box.
Rather compare today's PC clusters to today's mainframes. The mainframes still have them beat for high end data crunching.
Very few mainstream businesses need that much power. They generally don't need a PC cluster either except that the average single-machine PC-based database (especially running Windows) has a down-time rate that most companies won't tolerate.
When Microsoft will provide an iron-clad contract guaranteeing 99.99% uptime for a stand-alone system (like Sun, HP and IBM will) then PC clusters for businesses won't be necessary.
Since that won't happen, we will continue to see PC clustering being pushed by the Microsoft legions. Of course, the fact that a cluster of PCs requires a license for each Windows box (and each Microsoft SQL or Exchange package) has nothing to do with that, right?