A big reason for this is that Wikipedia's source is often a book or a journal article that is either offline or behind an academic paywall. Checking the source is effectively impossible without visiting a college campus's library. The likelihood that the cited information is wrongly summarizing the contents is low enough and the cost is high enough that doing so regularly would be irrational.
A bigger problem in this respect with Wikipedia is it often cites secondary sources hidden behind an academic fire/paywall. It very often cites review articles and some of these aren't necessary entirely accurate.
