Don’t try to be smart. We aren’t living in caves.
It isn’t the only reason. There are 3 reasons.
Solid Snake is one reason. Not Raiden. Not Big Boss. Not Venom Snake.
“And the word Solid in Metal Gear Solid has three meanings. First of all, Solid Snake is a character name.”
– Hideo Kojima, 1999 interview with Steven Kent for Gamers Today.
“This time Metal Gear is displayed in full polygonal form, and I used “Solid” to describe the cubic structure. also, the “Solid” means to the third power mathematically”
– Hideo Kojima, Metal Gear Solid: Official Mission Handbook.
Speaking of three-dimensions, it was also a sort of jab at Final Fantasy series developer and publisher Square, now known as Square-Enix.
“[…] you know, the company that does Final Fantasy. A square is a two- dimension thing. And I guess the president of Konami wanted this game to surpass Square. They wanted to make it a cube, you know, like solid 3D. So it’s got that meaning. too.”
– Hideo Kojima, 1999 interview with Steven Kent for Gamers Today.