Terrorist are good at surprised attacks on civilians, not so good against trained solders (with air superiority).
Israel is on a complete war footing and they could stop attacking Gaza and turned their attention to Lebanon, Syria, or Iran without much trouble.
It can not be lost on these leaders that Israel has been systematically taking out the leadership of Hamas. If they join the war, they (the leaders) become targets.
Every institution’s prime directive is to perpetuate itself and Hezbollah’s leaders are sleeping on piles of cash since sending money to Hamas is like setting it on fire.
The one exeception is the US Congress whose prime directive is to utterly expend all hope of future wealth and prosperity before it’s septuagenarian or older demigods die.