Basic risk vs reward.

You're not going to compete, so what are your options? Don't inform them and just don't show up?

No risk, no reward.

You inform you can't attend for ___ reason? Small chance of making a contact. Maybe they want you to attend next time. Maybe that person has connections for an internship or job.

No risk, potential reward.

There's a very small chance of any benefit at all tbh, but there is literally 0 risk. If your entire life is taking minimal risk to maximum reward, you'll do very well, whether you stick with CS or not. Most of the time you get no reward, but again you're taking no risk.

Flip the scenario to you risking $0 for $1k and you would call me crazy for not suggesting you take it. Sending an email is a 0 risk, potential reward scenario. Send it, you have nothing to lose.