* WhatsApp only produced large amounts of money for the founders but never profited and was intentionally never priced for massive profits.
* Messenger's monitization efforts have only been experimental so far, and those experiments have been lackluster from a revenue standpoint.
* Slack's financials are private and their most recent revenue claims that I have found are two years old but only $30M which would still be substantially far away from the billion dollar range even if they were in the 99th percentile of growth rates for VC backed companies.
In fact, industry estimates are that the market for messaging is growing but still <$10B. When compared to the sort of money that a single company makes from advertising on their search engine, some might say that messaging is still producing very normal human levels of money and nowhere near ungodly territory.