In my experience, the further you move away from the user and toward the hardware and fundamental theoretical algorithms, the less true this becomes.
This is very true for an email client, but very untrue for an innovative 3D rendering engine technology (just an example).
I would be surprised if there are more working email clients out there than working 3D engines. The gaming market is huge, most people do not pay to use email, hobbyists love creating game engines.
An email client is highly nontrivial, due to the complexities of the underlying standards, and how the real implementations you have to be compatible with don’t strictly follow them. Making an email client that doesn’t suck and is fully interoperable is quite an ambitious endeavor.