You can think of lots of reasons why, but it mainly boils down to this: VCs are hired to make bets within an asset class (startups) and not outside that asset class. Institutional investors (upstream of VCs) have other managers to do commodities etc.
Also, at least in theory, the market price of oil takes all info into account. If the VCs could really predict the price of oil in 3-5 years, they should probably be trading that alpha instead of investing in VC.