Equal cars? Who has equal cars? Can you imagine the following conversation taking place in any of the teams?
"We've got this great idea. We've tested it in the wind tunnel and in the sim and it gives us more than a second a lap."
"A second a lap? Wow! Okay, let's put it on the cars immediately!"
"Ah. Well, we'll only get enough parts manufactured for one car by the next race weekend. Shall we put it on our lead driver's car?"
"Only enough parts for one car?"
"Then, no. We wait until both cars are totally equal, even if it means losing a nearly guaranteed race win. It wouldn't be fair to our explicitly number 2 driver otherwise."
"Are you sure, sir? I mean, the sponsorship boost from winning a race, alone, will be worth-"
"I said, It. Wouldn't. Be. Fair."