My point is that we don't know that he keeps going. For all we know he runs into the exact same issue Vegito runs into.
we see Vegitto go SSJ2 (I thought it was 1, but there are the signature SSJ2 lightning) and last less than Gotenks SSJ3. without further evidence I am going to assume Gogeta SSJ2 at least lasts as long as Gotenks SSJ3 so that means he outlasts Vegitto
Vegitto separates and leave people in base form, so that means that the moment they break out, Gogeta only needs like a second to kill them
Also, your math is beyond screwy.
my math is beyond screwy because I was being GENEROUS to the Buu absorptions, for the sake of simplicity
it is just not as good as fusion, potara or otherwise
let's say that UG > Super Buu > SSJ3 Gotenks. which most people would agree, or at least have not a lot of problems with that
hell, let's say that UG = 2x SSJ3 Gotenks. I think this is their max difference. actual power is probably closer, and we will see why, but follow me here
it follows that UG is around 1.5 Super Buus. maybe a little more, maybe a little less
if Buu absorption is simply additive, that puts Buutenks at (roughly) 1.5 UGs. which is enough to turn the tide -which is what we see, Buutenks starts beating UG- but it is clearly not a big jump -Buutenks doesn't roflstomp UG-
which is why UG cannot be A LOT more powerful than Gotenks. if UG was, then simply absorbing Gotenks would not present a problem for UG.
it is only where the power levels of UG, SB and SSJ3Gotenks are close where the fight can go back and forth like what we saw. if you assume that UG and SSJ3 Gotenks are closer in power, then you also have to accept that Buu absorption is not 100% additive and some power is lost in the process. if you assume that UG and SSJ3 Gotenks are distant in power, then the Buu/Gohan fight cannot work out like it did. Buutenks would have made no difference if Gohan outclassed the kids by much. the characters themselves don't think that UG and SSJ3 Gotenks are too distant either
which also means that Buuhan is also not _that_ much stronger than UG. it's less than 2x, which means that Vegitto surpassing him is not that big a difference