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
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".
Basically it means to not have a special designed hardware for task X but to do much of it in software which gives you more flexibility. And also let's you configure and use X a bit more flexible.
E.g. software defined networking: If you run several virtual machines on a server, you may define the whole network between them virtually in software instead of doing it on the hardware side. Sure, you still need an ethernet card in your server to connect it to other servers and the internet, but all load balancing, switches, firewalls, VLANs, etc. between the virtual machines (or containers) on your server are virtualized in software - or maybe eben between servers.
Same goes for e.g. Software Defined Radio. In the early days you had dedicated hardware to control the mobile network and the antennas and such. Today you "just" have the antenna and a transceiver that is capable of producing and receiving a wide range of signals and modulations. All encoding, decoding and interpretation the signals is done in software. If your hardware is capable enough, the upgrade from e.g. 4G to 5G may only be a software update for all base stations.