A couple of reasons: 1. Doing so would be technically nontrivial. With current technology, as I understand it, the CPU is pretty much at the mercy of the motherboard. It can ask real nicely for conditions that suit it; but if the BIOS says jump, it jumps or crashes trying. I think Intel did try something with their CPUs or chipsets to make overclocking harder; but it was a matter of weeks before those tweaky OEMs in Taiwan worked their nuclear magic and started pumping out the cheap overclocker's boards we all know and love.
2. Doing so by legal means would be pretty draconian, and probably upset OEMs and customers, and give Serious leverage to their competitor. I imagine that, in these dark days of "DMCA" this and "Proactively protecting IP assets to build synergy and grow shareholder value" that, someone with sharky enough lawyers could ink a contract that effectively forbids any OEM from building an overclocker's board that works with your CPU, and makes any individual who modifies with intent to overclock a felon. (I jest; but only ever so slightly). This would pretty much just mean that the entire enthusiast market, and their suppliers would give the company that did that the single finger salute, so it wouldn't be worth it. Also, "accidental" leaks of "Unoffical testing BIOS versions" would probably become much more common.
3. Overclocking is actually a useful method of price descrimination: Basically, overclocking allows users to trade risk for performance instead of trading money for it. The sorts of people with high risk tolerance are usually the ones without too much money(the plutocrat overclockers who work with highest end gear are an exception; but they don't matter. Those guys buy the most expensive stuff possible, and sometimes kill it, so their overclocking obviously isn't hurting any bottom lines). Since overclocking is a risky activity, at least in that it voids the warranty, it already excludes pretty much the entire boring business box crowd, all real servers, mainstream systems from just about any OEM big enough for custom parts, people who don't care, etc. This is most of the market. For the part of the market that does overclock, overclockability is basically a method of providing a discount to enthusiasts in order to compete more effectively. For example, there was that period a year or two back where AMD had Intel pretty much beat, except for the fact that their low end P4s could pretty much always be wound up to match their top end; but for much less money. Because the Dell set doesn't overclock, the cheap 2.4s, or whatever they were at the time, didn't cannibalise sales of 3.4s. They probably did prevent the sale of a fair few AMD chips, though. Obviously, the specifics of the economic analysis vary sharply depending on what is on the market at the moment; but the basic fact of price descrimination remains.