Fleets auto-withdraw rather than fight to the death if the space forces ratio is stacked against them. However, unescorted transports can refuse to perform invasions in situations where they could easily get to the ground. For example, if a planet has no infantry on it, only one transport needs to land in order to take the planet. However, if the planet is protected by like 30 starcruisers, a big unescorted force of transports may refuse to engage even if there aren't enough opposing ships to destroy all the transports before some get to the surface.