So, you ask, what about Silicon? Silicon also has 4 valence electrons, so it ought to be a nice substitute, right?
The problem is that Silicon is in the next row down.. so it's more massive and it's electron cloud takes up more room. So you still only have 4- connectors, but it's a larger element.
There's clearly an advantage into being able to form precise and small molecules. Carbon, with it's small size and 4 valence electrons, is a clear choice for a backbone.
I'm a complete layman, but how does that play out in planets with gravity force significantly stronger or weaker than Earth? Woul that skew which elements make the best building blocks?
Silicon is basically impossible as a building block for life. It forms bonds that are too strong, meaning it requires much more energy to fuel life processes and reducing the rate of chance collisions which lowers the prospects for abiogenesis.
Carbon is the only realistic choice; it is the only element abundant enough with the right balance of stability and versatility.