The problem is that its so expensive to build from scratch. All Unity does is build just the engine, and that's enough to make it a 7000 person company. Trying to build a game engine and then an actual game on top is a herculean effort.
This is why open source software is so important. It enables these small companies to pool their resources and share an engine as long as they each contribute fixes back.
7000 people is misleading. Being a general purpose game engine it has to be everything for everybody. An engine developed for a single game can be simpler, and once it is done, making the game will be simpler than it will be in Unity. Also those 7000 people are doing way more things than develop an engine.
That said, an engine like Unity can save a massive amount of time, especially for games that are medium scope. It's these games where developing engine code and tooling would both take a lot of time and the advantages would likely go unnoticed.