Twin engines cost rather more for equivalent size, and design around the a
end of the aircraft is trickier.
Most twin-jet fighters - the Jaguar and F-5 are exceptions - are twins because there was no fighter engine big enough to do the mission with a single when they were designed. (Even the classic Bug had 32k thrust at service entry, much more than any modern fighter engine of the day.)
Dave is a single because it's STOVL. Making it a twin would have been insanely complicated. Also, unless you can sort out the asymmetric issues in powered lift, an engine problem in STO or VL is more dangerous with a twin than with a single (because it will turn upside down very fast) and with two engines it is twice as likely. Very early on, the Navy wanted their version with two F414s, but "studies" were done which "proved" one engine was just as good.