I'm bad at low-level networking, but could it be a routing issue? Effectively every machine is also a router, so there might be some wasting going on.
At 10gigabit/second, with 1518 byte packets, it's 823451.9104 packets per second.
So in a single stream, you have to process each packet within 1.21microseconds to keep up
At 20gbps, you have 600nanoseconds per packet.
There are also almost certainly timing/synchronization issues between different stacks like this. It's horribly inefficient.
Network cards achieve >10gbps in part by offloading a lot of stuff.
Even if the CPU can handle the load, just going through different stacks like they are may add enough latency to throw single stream throughput off.
The posited reason of "not compete with network cards" is beyond stupid. It can't because you can only do a few meters this way, max.
That's not interesting at all. 25gbps network cards are cheap and for a few meters, a $10-15 25gbps DAC will suffice
For more than that, 25gbps transceivers are 25 bucks a pop for 25gbps-SR, which will do 100meters.
With none of the problems of trying to use longer thunderbolt cables, too.
Intel's 25gbps SKUs are not where they make their money anyway.
Why did they set a limit so low is not known, but the supposition made by another poster that this is a market segmentation feature may be right, because such policies have always been typical for Intel.