Part of the problem is that most 401Ks have a very limited set of investment options. You get a short menu of choices arbitrated by a clueless HR person at your company and the self-interested financial company that manages the 401K plan: a bunch of standard mutual funds and index funds, some are semi-targeted (e.g. large-cap vs small-cap, whole-market, foreign-vs-domestic, target-date, tech?). If you're lucky, there might be a couple of different bond funds, and maybe a money market option to park cash temporarily. There's not often the flexibility of a regular investment fund (e.g. an etrade account) to go after other specific options or individual stocks and bonds, or especially any kinds of commodities, futures, options, etc.

I get the rationale: it's set up that way so that it's a "safer" investing option, because they're afraid people will make bad choices and lose their 401K balances. But the flipside is you don't get much flexibility in making the wisest or most self-interested investment decisions. The 401K -managing firms love it though, as they get to sell a bunch of funds that are often in-house and have fees, and the real free-market-trading investors like it because the 401Ks put a bunch of very predictably-timed money into predictably-common choices, and they can rely on this to gain a little advantage/arbitrage.