Why doesn't the transfer common app let you designate that you did an activity in 9th or 10th grade?

As a college freshman this is VERY frustrating to me.

I really don’t want the adcoms to think I only did things in 11th and 12th grade.

Don’t think too much into it. They’re familiar with the common app and know that 9 and 10 aren’t options. Things done ONLY in 9th and/or 10th aren’t worth reporting and if something was a 4 year activity you can reference it in the description if you want. Also, many colleges will accept a resume (totally optional) if you feel that the activities section of the common app is not enough.