Probably because the English words "I" and "us" are not derived from Latin (they're derived from Middle English), whereas the plurals "alumni" (male or mixed gender) and "alumnae" (female) the singulars "alumnus" (male) and "alumna" (female) are Latin words that have become included into our language. (You knew that )

None of this, however, would explain why we PARK in a DRIVEWAY and DRIVE on a PARKWAY... or why a CAR may carry a SHIPMENT, where a SHIP would carry CARGO....

You're right, Jim... we DO speak a funny language.