It has to be easy enough to move the Bitcoins that it can be done when needed. For example, if you split the password up among too many people, then you could “disappear” one of those people and then the Bitcoins suddenly cease to exist (or practically so).

So there’s a balance. If it’s easy to move the Bitcoins, then a thief can find a way to move all the Bitcoins into their own wallet. Or, if it’s very difficult to move the Bitcoins, then an adversary can find a way to make it impossible to move the Bitcoins, essentially destroying them.