Two reasons, and one is more important than the other.
The first reason: 5 Sparks are enough to make people pause before firing off requests in bulk. We want connection requests to feel deliberate. The recipient should be able to assume that the person reaching out actually read their profile and thought about it for a second.
The second reason: the 5 Sparks are refunded if your request is rejected within 7 days. That's important. We're not penalizing rejection; we're filtering for sincere interest. If they say no quickly, you get your Sparks back. If they sit on it for more than a week without responding, you keep the connection request in flight, and the Sparks stay spent (it incentivizes choosing thoughtfully).
This pattern keeps the network calm. People who get a Cofelt connection request know that the person on the other end meant it. The signal-to-noise ratio stays high. The cost keeps requests considered, which keeps the network less noisy for everyone.