With Postgres, you can do almost everything, also a full-text search, but you still have Elasticsearch, Mejlisearch, etc when you need performance and advanced features. The multitool approach is suboptimal in most cases.
In small teams, the infrastructure is often not able to be fully utilized, so performance is not an issue. However, feature richness allows this team to deliver higher-level feature faster. Think early stage startup (one or two engineers) or hairdressers-like business (they use a ready-made framework that targets a popular database and limits its feature to have a wide range of users). As a result, you can have a lot of such business creating a very long tail.
SaaS products are infrastructure. Each different SaaS used is another piece that needs to be connected to the system and maintained; it thus becomes part of the system infrastructure. Each new SaaS piece has costs (time and money) associated with it.
That said, it's up to the individual company to decide if the added cost is worth it. Just because the cost exists doesn't mean it isn't worth it.