The WMF balance sheet for 2015-16[1] shows that they held $46.7 million in cash and cash equivalents. Long term investments are listed as $11 million.
Fun trivia from their Statement of Activities[2]: they spent more on processing donations ($3.6m) and conferences/travel ($2.3m) than on hosting costs ($2m).
[1] https://upload.wikimedia.org/wikipedia/foundation/4/43/Wikim...
[2] What a for-profit company might call Income Statement or Profit & Loss.
I can see how you'd come to that view.
But there's no need to guess, here. These terms have legal meanings derived, in the USA, from the GAAP standards. "Cash and cash equivalents" doesn't mean "actually it's all cash because we also listed 'short term investments'".
So, looks like their annual budget needs that cash. From the OP's chart, expenses are increasing exponentially, maybe their budget is continuing that pattern. We also don't know how lumpy their donations are. Maybe it all comes in during their donation drive.
To answer your specific question, I think Wikipedia is not well funded. Many museums and foundations are funded for years to come, so they can take a bigger proportion of risky assets. Wikipedia seems to spend donations as they come in. Needing 50% of their assets liquid highlights what the OP is complaining about - they spend too much. They are simply unable to invest more long-term.