Still no US President?

It’s been 4 days since the election and the election still hasn’t been called. Why? I understand they want to get this right by counting all the votes. This years’ election was different as many mailed in their ballots. However, I don’t recall an election taking so long to call. Clearly Biden is ahead, in many of the battleground states, so why not just call him the winner? I have done the math and I don’t see a path to a Trump victory. Something smells funny here.