Yeah many people are playing custom games, the lobby doesnt really show many games at a time but think cause the games start pretty quick. Its probably quicker than spending 1- 3mins searching for a rank game
https://www.ageofempires.com/stats/ageiv/
Theres over 100k players on the custom games ladder, Not sure how the ranks work
And if you want to closest to a even team rank game 2vs2 has the most people playing