Why?
To clarify, why must this point of view be fundamentally unsatisfying?
Let me elaborate.
Not only is it unsatisfying from the perspective of knowledge and understanding, because once God is invoked, there is no more intellectual incentive to further investigation. It's just something that is, happens, for reasons we don't understand and don't care to learn about. "God did it" is the end of cognitive process.
But in addition to that, it is exceedingly dangerous as well, in many ways.
Using the popular statement "God works in mysterious ways" as an example, to me it exemplifies the problems associated with predeterminism. It seems to me like it encourages fatalistic, apathetic attitude about events occurring in the world and in our lives. It is not useful in any way to predict anything. However, it can be used to justify anything and to support any agenda you might want to.
Second reason I find this outlook of life especially troubling is that even if there
is a God, and even if he
does have a mysterious plan, there is
no guarantee that the plan is beneficial to us.
Everyone just likes to think that God's plan will favour them personally. God bless the USA, eh? Why should God bless the USA, it only has a fraction of people living on Earth. It's the Promised Land of Israelites all over again - why should they be God's chosen people out of all the people in the world?
Considering the amount of differing agendas and factions on the world,
God's Plan cannot favour everyone. In fact, there is
no indication that "God's Plan" favours humanity at all.
To paraphrase a certain well-known fictional character:
Nobody panics when things go "according to plan." Even if the plan is horrifying! If, tomorrow, I tell the press that, like, a gang banger will get shot, or a truckload of soldiers will be blown up, nobody panics, because it's all "part of the plan".Well, when
everything is part of "God's Plan" (because it's
categorically defined as such) then clearly there's no reason to panic because we can't really do anything about it... even if in reality there might well be good reason to panic, and start doing something about things that aren't going so well.
This can very easily lead to completely irrational decisions and actions, causing potentially irreparable damage and loss of life.
Examples of such logic in action:
-vilification of birth control as it goes against God's Plan (ignoring the fact that maybe birth control was also God's Plan)
-refusing medical treatment because life and death are for God to decide, or worse yet forbid their children to be treated
-ignoring the effects of releasing hundreds of millions of years worth of accumulated carbon into atmosphere within a few hundred years and expecting Earth to be just fine about it
The last example is not necessarily always religious in nature. However, "All Part of God's Plan" logic seems functionally identical to the reasoning that assumes humans to be too insignificant to meaningfully change the Earth and therefore Earth is fine regardless of what we do.
TL;DR: "God Works in Mysterious Ways" reduces human independent agency to such an extent that it doesn't even matter any more.
Addendum: I know that people's brain work differently and we perceive things differently. There are strong indications that religiousness is something that some people have a neurological, likely genetic predisposition toward. I'm not trying to devalue the way religious people perceive the universe, but it also has the potential to cause tremendous problems, as with every case where people make decisions based on things that are not necessarily true.
If I choose to make my decisions in life based on something I believe, I would like to know if my beliefs are true or not, because if they're false I might end up making poor decisions.