Combining multiple network links into a single usable link is something the industry is very slowly moving towards.

Multipath TCP is already a thing and is used extensively by iPhones at least for Apple framework services.

If you have multiple network connections, you want to use a multi-path VPN to bond them together to allow you to aggregate bandwidth or reduce total latency or reduce packet loss.

As somebody else mentioned speedify is a commercial multi-path VPN service you can use today on any device.

You could also build your own if you're so inclined, but wire guard does not have multi-path built in you have to bod something together. There is a patch for open VPN for multiPath, but it is not in the upstream

We can and have. It’s Multi-Link Operation (MLO) which was part of WiFi 7 standards. Here’s how it works.

My router has it and it is indeed quite fast, but it’s new technology. There are currently very few client devices that support MLO.

Because it's not useful. Two routers still share the same frequencies and thus can't send more data over the same air. A single router can already use multiple frequencies to increase throughput. You don't need two to do that. https://en.m.wikipedia.org/wiki/MIMO

This exists, kind of

There’s bonded connections in several senses

Bonded ports but this doesn’t increase throughput in the way you’re thinking. eg if I bond 2 1 gigabit Ethernet ports I can’t connect at 2 gigabits, I can connect 2 users at up to one gigabit each (or several users totaling 2 gigabits but no 1 user at more than 1 gigabit)

bonding routers can take two internet connections and combine them, which is closer to what you are probably imagining. They combine throughput, eg a 100mbit connection and a 100mbit connection become a 200mbit connection although realistically it’s not that perfect and you have to get the right services for it, not just any connection will work, it’s a rabbit hole and generally much slower and worse latency than if you just got a traditional connection. Think people using starlink and 5g internet in rural settings

There’s also something called speedify, which is software that claims to do the above in software alone, bonds two connections to combine throughput. Never tried it, reviews are mixed. Some say it works, some say it’s spotty, some say you only get the speed of the one connection, etc.

We had a guest speaker from ericson back when I was in uni. According to them that's been a thing for a while now

Just duckduckgo/mojeek your question. I got link to speedify for example.

We've been doing better time sharing since WiFi 6. Remember this all has to be backwards compatible.

WiFi 7 has its own new band and its really fast.

My guess is that a network card can handle only one network at a time

probably because it’s more complicated than just improving the bandwidth on single wifi networks, which we have been making steady progress on. picking the low hanging fruit first.

I wonder if you configure them to be a trunk