Unity employs a small army (about 3K) of senior software engineers, that can definitely command upwards of 200K per year, which puts the estimate at 600 million, plus a number of specialists (mathematicians, physicists) for things like cloth and hair simulation, large water body behavior, etc. They maintain a large compatibility for a huge and growing list of varying hardware devices, phones, consoles, this means they have to get things like floating point operation results reliable on all machines, older x86 processors, RISC chips, etc going back several decades, and even get experts directly from places like Microsoft for features like their C# to native compiler. Most devs don’t appreciate just how much commercial game engines handle in the background and make your build process so much easier. It’s definitely would take a slew of specialists years to get something remotely comparable.

Godot on the other hand while a highly capable engine, is a much lighter weight as by design (Juan L says so himself on the Godot blog). Features like plug and play multiplayer servers, machine learning for NPC behavior, encryption for credit card IAP, either aren’t included or havent been implemented as heavily or are only included in the asset store, as a product of a volunteer contributer, as most FOSS software is known to have. Just going through the unity package manager will show off the disgustingly bloated pile of software that is the unity engine. Probably several tens of thousands of packages available that most people will never use.