It's a good point by ChoChikun that the BB's hand is not random after many players fold. The players who folded are more likely to have folded low cards than high cards, so the remaining deck is richer in high cards, particularly aces. This is called the "bunching" effect. This is ignored by programs like SNG Wizard, ICM Trainer, or the Nash calculator on HoldemResources.net.
The main reason the bunching effect is ignored is that it is easy to ignore it, and very hard to include it, particularly when you are trying to compute the push/fold Nash equilibrium. If you can ignore the bunching effect, then to compute the Nash equilibrium for 4 player, you can start by solving blind-versus-blind, then see when the button would be better off pushing, then add the CO. If you have to take into account the bunching effect, you can't build the solution this way, since you don't know yet what the blinds have when the CO and button fold.
Many people hope that the bunching effect is negligible in Hold'em, and there are reasons why it is smaller in tournaments than in some of the simulations for cash games. When there are fewer players at the table, then the bunching effect is reduced. Also, people may play fewer weak aces like A2s or A5o in early position when the stacks are shallow than when the stacks are deep.
If the chance to run into AA is 20% higher than if the BB had a uniformly random hand, the chance to run into AT might only be about 10% higher. AA is an extreme case.
As noclaninator mentions, the bunching effect suggests pushing a tighter range than you could against someone with a random hand. This is just one effect left out of the usual models. There are others, such as that players don't call with a consistent range: If you get called 15% of the time, it won't be by the top 15%. You will see AA, JJ, AQ... and some surprisingly weak hands like 98o. The inconsistency of your opponents lets you push wider.
While the bunching effect may discourage you from making some marginal pushes from the small blind, it may encourage you to push wider from earlier positions when you have a hand like Ax. The Nash calculator sees that when you push an ace, then there are only 3 aces left in the deck. However, when you fold an ace, it assumes the ace is shuffled back into the deck. One reason to fold is that you hope your later opponents will collide. However, if you have Ax, it is less likely that two of your opponents will have a pushing and a calling hand. So, folding A2 is not as good as folding 32, and you might be right to push A2 when you wish you could change your hand to 32 and then fold.
It will be a step forward when programs can incorporate the bunching effect in their analyses, and can tell us whether the bunching effect is significant or not.