Checking pockets isn’t enough. Pitchers have been hiding the substances on their hats or gloves or other sneaky places for years. It’s not like they just bring a jar of spider tack out there with them
To be totally clear, you could do this, but it's going to start an arms race of trickery.
Watch an MMA match. It takes all of five seconds for a ref to run their hands over the competitors head, hands, etc.
Which is certainly better than… nothing.