Because the BBEG never has minions between himself and the party. He never has allies with ranged attacks to take out the PC spell caster. He is always within 30 feet of the grappling Fighter at the start of the encounter (whose init beats the BBEG's). He never throws spells right away that hinder the PCs or have allies do so. The PC actually prepped Silence this particular adventuring day, or didn't swap it out because he never got to use it as a class that doesn't prep spells and finally gave up on it. Life is always ice cream and flowers and puppy dogs for the PCs. Woo hoo!
Sorry, but your scenario sounds contrived and optimistically simplistic.
And you're interpreting my scenario in such a way that you can be pessimistic. Where did I say that the silence/grapple needs to be in the first round? Or that it has to last indefinitely?
Also it's far from the only scenario to maximise silence in combat - how about a Sentinel melee next to the BBEG instead of grappling? In that case he could even dash to get next to the enemy spellcaster and not need to use an Action to facilitate silencing him.
Any caster who has Silence should always, always have it prepared. It has innumerable out-of-combat uses in addition to stopping spellcasting - which is really more of a niche role considering how many fights won't involve enemy casters. Silence is one of the very best utility spells of its level in the game, right up there with Invisibility and Suggestion.