The universe as a hologram is a pretty inescapable conclusion when you look at the cold hard physics. The reason Susskind and others say the 3
dimensional universe is an illusion is because it has to be. There's really no way around it.
What things like black hole thermodynamics and the holographic principle tells us is a 3 dimensional objective reality doesn't exist. Susskind calls
it an illusion.
This is because physics gives us a limit or the maximum entropy that can occupy an area or surface boundary. An objective 3 dimensional object of
matters exceeds this limit. When I'm looking at the cup next to me, physics tells us that if this cup was an objective 3 dimensional cup made of
matter it couldn't exist. It exceeds the maximum entropy or the surface boundary it occupies and would collapse into a black hole.
What people like Hawking and Bekenstein showed was that this boundary corresponds to a 2 dimensional surface area 1/4 the size of it's volume. So
information on this 2 dimensional surface area projects the 3 dimensional world that we see because again, physics tells us that an actual 3
dimensional cup or car made of matter would exceed this boundary and collapse into a black hole.
Let's say you fill up your closet with boxes, you would think the volume of your closet determines how many boxes you could fit into your closet.
This isn't the case. How many boxes you can fit into your closet is determined by a 2 dimensional surface boundary 1/4 the size of it's volume. What
this tells us is that the boxes that you're putting into your closet have to be a hologram or a phantasm because if they were a 3 dimensional
objective reality, you would create a black hole in your closet because the boxes would exceed this limit or exceed maximum entropy.
I think the heart of the holographic principle is pretty simple and straightforward. It just goes back to Plato and the cave. He said when you tried
to tell people the objective reality they experience is an illusion they will not be able to handle it. This simple description from Wiki sums it
up:
The holographic principle states that the entropy of ordinary mass (not just black holes) is also proportional to surface area and not volume;
that volume itself is illusory and the universe is really a hologram which is isomorphic to the information "inscribed" on the surface of its
boundary.[9]
Here's a video of Brian Greene and Susskind talking about this: