SQLite is used for small and embedded systems, which (if successful) can have very long lifetimes. If you were building something like an ATM for instance, you would be very sensible to sign a 35-year support contract for a crucial part of your system.
Microsoft has nowhere near the quality, security and bug-fix track record, documentation coverage, nor testsuite coverage that SQLite offers. There's simply no way they can possibly provide such support.
There are products that Microsoft offers long support contracts for. To give an example, EOL for XP Embedded is in April 2019, which means XP in all its forms would've been supported for about 18 years.
