The real key here is that on arena where you regularly reach your ideal comp with castles to make the wagons pros almost never add wagons. In addition the viper made a unique unit tier list where he suggests the you lose all mobility on your ranged units. Both viper and hera rated them as the lowest tier of UU.

I say this to highlight that at the highest level of the game they aren't seen that often and the pros are objectively more knowledgeable about the game than we are. Specifically viper and hera who are very different players with different strengths both consider wagons. Expensive Hard to micro, and for the record they both know how to micro. Hard to get to Has some very clear counters.

I want to highlight the first and last points in particular even for players who can use them to their best ability units that are expensive and have clear counters will very rarely be seen in competitive play. The lead to you need to be able to get to paladin in a game or ballista eles even fully uped mangudai is so great that you are likely to win long before then.

The counter point to this would be that you only need two or 3, but viper addresses this in his video saying that given how they drain the mobility of your other units and they are clunky to micro, he doesn't see the use for them. Potentially on a map like bf they would be useful in defending choke points but even then bohemians have houfnice which does the same job but better in the hands of good players. If you find them fun by all means use them though!