There are a lot of reasons for this, but there are ways to deal with them, like a pre-engineered hanger or bracket.
Short answer is Physics and Engineering principles.
Longer answer is this:
Using a bolt, vs setting the joist edge on a hanger or top surface of the post means that the forces are transferred through the bolt. for the joist that means that the effective depth of the joist is at whatever depth you have drilled the hole. If it's the center, then your 2x10 just became a 2x5 effectively. (Think similar to notching the end of a joist)
Also the bearing surface became MUCH smaller, instead of having 8 square inches of surface for all that force to transfer through (width of joist on top of the width of the post) you have (I am estimating here based on a 1/2" thick bolt) .25 square inches (thickness of joist x bearing surface of bolt) now all your forces are being transferred through a surface that is 1/32 of the size. Now you are getting into the crush limits of the wood fiber.
It also results in forces bieng altered. If you have the bolt in the center of the joist then there might be an induced twist, and bolts can and will bend, reducing their strength. Also, twist on the joist moves the load off axis rfirther reducing the capacity of the joist, and inducing a different loading at the end of the joist. All of which reduce the capacity of your joint, and joist.
So the chances of your joist splitting at the bolt, and the hole getting bigger because of crushing, you no longer have a stable joint. Wiggle means dynamic forces and shock loads. Its just a matter of time before it fails.
Caveat: I estimated the bearing surface of the bolt due to the surface being curved, and forces transfer normal to the surface. So your vertical force component won't be evenly distributed across the entire curved surface. So it could be more, or it could be less. For the purposes of the answer the exact amount isn't entirely important, just the fact that it results in a much smaller bearing surface.
Longest answer gets into static and dynamic load analysis and involves a crap ton of math that you need an engineering degree (and a cheat sheet) to remember, and involving a material with hugely varying characteristics (type of lumber, moisture content, age, grain, cut)