1 Word:

Regeneration

You should be able to understand now

ohhh. so basically battle until battery runs out if using cb team

Not exactly. The reason unkillable teams work on CB is because you can plan out your turns with a very high degree of precision. That lets you set up cycles to ensure your whole team is always protected by either block damage or unkillable buffs.

It doesn't work on Spider because the spiderlings completely mess up your rotations - not to mention the Queen's spiderling feast resetting your TM. The ticks will cause your champs to lose sync on the buffs, and you'll end up in situations where your block damage/unkillable falls off a champ before they can be refreshed.

but wouldn't something like a 3 maneater, 2 septimus team work? Does block debuff block the tm? If not, why not just build high resist?

As Maneater has a full turnmeterreduction on his A2, this team could indeed work at Spider. 

If you ask why there are not more teams consisting of this champions out there: How many players do you know, that have 3 Maneaters and 2 Septimus? I guess you know exactly zero players with that champions, what probably is the answer to your question.

A typical CB unkillable team (that has less than 3 maneaters, lol) won't work at Spider, because you need turnmeter reduction to prevent the big Spider from healing and one Maneater with his A2 on a 3 turn cooldown isn't enough.

oh ok. so basically just a HH team