In the programs I write probably about 80-90% of variables are immutable, and I think this probably corresponds to most other code. Except in certain domains and programming styles, not that much stuff tends to need mutability.

This is why the syntax "encourages" immutability by making it the easiest option (similar to e.g. Rust, F#). On the other hand, if it was an extra keyword nobody would use it (e.g. like Java).