Deal size. A dinner for twelve is a meaningful cost once venue, travel and the time of the people hosting are counted. If your average contract is a few thousand a year, the arithmetic does not work no matter how good the evening is.
Committee size. The format's real advantage is reaching people your targeting could not name - the +1 who turns out to control the budget. If one person decides, you do not need a room; you need that person.
Channel saturation. Events get expensive and still win when the alternative channels are exhausted. If your cold email still books meetings at a reasonable rate, spend there first. Events are what you reach for when the cheap channels have stopped working, which for most enterprise segments they have.
Not leads. Three things:
Not the venue - the coordination. Filling a room properly means finding, for each guest, the colleague with the warmest relationship to them, briefing that colleague, and tracking who has asked whom. That is the work that makes the difference between a half-full room and a full one, and it is the reason programs quietly revert to one marketer sending one email, at which point the acceptance rate collapses and the format gets blamed.
Run one, properly, against a named account list, and measure the accounts rather than the attendance. Compare their behaviour over the following quarter against comparable accounts that were invited and did not come.
One well-instrumented dinner answers this question for your business better than any published benchmark - most of which are produced by companies that sell event services and do not disclose their method.