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.
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. en.m.wikipedia.org/wiki/MIMO
If you want to use multiple internet connections and combine their speed, that’s possible. Dunno how though and I guess to work best it would need a server somewhere else like a VPN to manage the packets coming from different ips
SD-WAN includes that but it is not its sole purpose, although I agree most vendors will say that’s what you want. WAN/Link Aggregation, Multilink Aggregation, Link Load Balancing, Equal Cost Multipath, WAN Virtualisation, etc are ways to bundle multiple links together.
In WIFI terms, it’s called channel bonding, it was proprietary and various vendors had their own implementations, see “Super G”.
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.