Because this is not a good solution for everyone. I recommended reducing the payload size only for this who discovered that a larger payload size does not work for them. Many can handle larger DNS buffers just fine. For instance, my local unbound handles 4096 bytes packets just fine.
Because this doesn't seem to be documented anywhere properly, I probed all the DNS servers currently offered by Pi-hole to find out their maximum DNS packet size:
Name
Address
Maximum packet size
Google (ECS)
8.8.8.8
1400
8.8.4.4
1400
2001:4860:4860:0:0:0:0:8888
1400
2001:4860:4860:0:0:0:0:8844
1400
OpenDNS (ECS)
208.67.222.222
1410
208.67.220.220
1410
2620:0:ccc::2
1410
2620:0:ccd::2
1410
Level3
4.2.2.1
8192
4.2.2.2
8192
Comodo
8.26.56.26
4096
8.20.247.20
4096
DNS.WATCH
84.200.69.80
4096
84.200.70.40
4096
2001:1608:10:25:0:0:1c04:b12f
4096
2001:1608:10:25:0:0:9249:d69b
4096
Quad9 (filtered, DNSSEC)
9.9.9.9
1232
149.112.112.112
1232
2620:fe::fe
1232
2620:fe::9
1232
Quad9 (unfiltered, no DNSSEC)
9.9.9.10
1232
149.112.112.10
1232
2620:fe::10
1232
2620:fe::fe:10
1232
Quad9 (filtered + ECS)
9.9.9.11
512
149.112.112.11
512
2620:fe::11
1232
Cloudflare
1.1.1.1
1452
1.0.0.1
1452
2606:4700:4700::1111
1452
2606:4700:4700::1001
1452
You see that several do allow for a packet size of 4096. Level3 even allows for insane 8192 bytes.