Because you have to be occupied before you can do guerilla resistance? Also because guerilla activities require fewer personnel than an armed mobilization?

If Ukraine fell to invasion, as in like their lines collapse and they lose the territory they’re currently holding, now every Ukrainian who might consider fighting is already behind enemy lines. Instead of being concentrated physically and very obviously defending themselves you’d have a very pissed off post-invasion population able to act inside of occupied territory. Look at the resistance fighters in occupied France in WWII. Putin isn’t going to have an easy time even if he manages to push an invasion force through.

Good luck holding a country that fights as hard as Ukraine.