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.
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”.