The problem is that with spontaneous life creation you don’t get a fully functional cow. You get a version 0.0.0.1 of life that is something like a molecule that (by random) can make a copy of itself every 6 days, but it is decomposed by the ambient in approximately a week, so it has a little chance to do two copies.
So, slowly but steady the amount of this molecule increase. Perhaps it needs a million year to conquer the word, but there is no rush because there is no one to compete. Some of the copies are bad, but one of the copies can copy itself in an amazing 5.5 days! (Or perhaps lives for 7.5 days and it has more time to try to do the second another copy.) Now the new version 0.0.0.2 conquers the word! ...
If something like that appears today a bacteria will it in a second, a amoeba will eat the bacteria, some nasty animal will eat the amoeba, and a lot more of eating will happen before the spontaneous generated barely alive molecule has time to do a copy of itself.
The problem with the second spontaneous life creation is that the first spontaneous life creation creature will be so much faster that the second’s creatures will have no chance to survive.