Having shipped large heavy speakers and smaller packages for over 2 decades, we have witnessed some real freight abuse and have learned a few things along the way. During the first couple of years importing Analysis Audio speaker we spent more time redesigning the shipping crates than on anything else. During one of the first shipments I witnessed the UPS driver slide the crate to the back of the truck and push it off the end. The crate did an end over end barrel roll. I couldn't believe what I saw. I had words with the driver and he still wouldn't let me help him with the second crate. The first crate was destroyed with the speaker exposed at one end. Miraculously the speaker was not damaged. It confirmed to me just how rugged Analysis speakers are. In good faith, after witnessing that abuse, I ordered the customer a new pair and kept that pair as a demo. Thankfully they never had issues.
To survive shipment we ended up using 1.25" plywood around the perimeter and 5/8" plywood for the large flat sided with cross-braces. Internally we use strips of high density polyethylene foam in strategic locations. The foam does not contact the entire perimeter or sides, only in spots. It's all about minimizing and distributing the shock loads transferred to the speaker.
Most stereo equipment is not designed and built to withstand any significant shock load so the packaging has to lessen that load. For example: Imagine a 50 pound amp with an 8 pound transformer in it held in place by 2 #6 screws. Now place the amp in a box sized large enough so there is 3" of space around all sides. Add 3" of very ridged hard foam all around. Seal it up and drop it from 3'. It's likely the outside will look OK but the transformer has detached. Now take the same amp (with transformer in place) and box and substitute the ridged foam with 1" thick softer foam then 1" thick medium foam and the outer layer 1" thick high density foam. Drop it from the same 3' in the same orientation. It's very likely the transformer will still be attached and outside of the amp will be OK. That said I wouldn't use 2 #6 screws to hold a 8 pound transformer but you do see lots of weird construction in audio. Foam being expensive, you don't see packing as described. What is more common is the use of high density polyethylene foam in strategic locations in varying cross sections. Think of it as damped spring system designed to minimize the transference of the shock loads applied by the shipping abuse. The box or crate has to be designed to handle the weight deal with intrusion.
Even with using substantial wooden crates, we almost always have to repair or replace parts after receiving our gear back from shows. The main thing is that the equipment inside is well protected.