The prefixes over- and under- (meaning, respectively, (1) too or (2) above, and (1) insufficiently or (2) beneath) are living prefixes, meaning they may be attached to virtually any adjective or verb without requiring a hyphen. Your spell check might tell you not to use words like underintelligent, overlove, undermarket, overroof, underfoundation, and oversunny, but spell check is often wrong in instances like these. These are perfectly good coinages that do not require hyphens.
Even so, because many people believe spell check is infallible, such words do commonly appear with the hyphens. The hyphens are not wrong, but they are unnecessary.
Although the over- and under- prefixes can be attached sans hyphen to almost any adjective or verb, many writers and editors are timid about doing this—for example:
Parents tend to over-treat fevers, even waking up sleeping kids to give them fever-reducing medicine. [CBS News]
Roth explained that the legislature first began to under-fund the program back in 1993. [KSAL]
Personally, I think times in the 40-yard dash are mildly over-rated. [The Virginia-Pilot]
Most of our kids are under-skilled. [Stuff.co.nz]
But there’s no reason that these hyphenated words cannot go without the hyphen, as they do in these examples:
It was neither necessary nor desirable for the H.M.O. to overtreat me. [letter to New York Times]
It would underfund school districts by 11 percent, or $231 million. [Bloomberg]
An overwhelming amount of prepublication praise from established writers has endorsed this impressively crafted, if cold and overrated, novel. [Irish Times]
It believes the evidence of its eyes, not word-play from dodgy politicians and underskilled public relations flacks. [Courier Mail]