Quantum physics deal with a different kind of phenomena that seems to be random in nature. Macroscopic physics emerge from micro-world, so different scale.
If what you are comparing is classical with relativity, Occam only applies to simplest answer that explain all observable reality. Classical didn't explain enough, so it needed to be corrected by a more complex system.
I am wondering what the reasoning behind that is.
Because it doesn't involve the assumption that an all powerful being exists.
The simplest answer is not guaranteed to be the correct answer.
I'm not sure why you are attempting to use Occam's Razor on a person's belief in God and it has the caveat of "usually". 2008 happened because most people accepted the simplest answer, for instance. Simple answers frequently fail in complex situations involving human interaction and a simple acknowledgement that the Bible was written by fallible men who may have misunderstood a few key details and the miracles were metaphorical rather than actual is an equally simple answer.
God isn't about who wrote the right book or got the details perfect, it is about knowing that at the end of the day you should be a good person who loves your fellow man. We will all be judged in the end, even atheists admit as much if they thought through what happens after they die (by their fellow men, if nothing else).
There's a huge difference between knowing people are going to judge your story (at least the story they think they know about you) and believing some all-knowing being is going to choose where to put your soul based on every single thing you ever did or thought. One idea makes sense and has plenty of evidence to support it, the other is bat-shit crazy.
Just because you feel other people are "bat-shit crazy" for not sharing your belief system doesn't change the fact the beliefs are based on a valid life lesson.
I'm curious, do you believe everything you read is literally the complete truth without some interpretation being required? Do you believe everything a politician tells you as the literal truth?
When you are faced with something that can not be explained by the principles you understand, does that mean that the phenomenon ceases to exist ?
So, by that line of thinking - the diffraction of light could not be explained until we had a new theory about light being an electromagnetic wave, so should we have ignored the experimental results altogether, because it's more convenient to what we can understand?