We get very busy and to quickly sign-in clients, we only want their first name and phone number as required fields. These two pieces of info are enough of a differentiator that we don't want to spend time asking someone to spell out their last name. Some people have very long and complicated last names, and we would rather not have to get their last names wrong and have it be blasted across all marketing and text reminder communications every single time, and instead just use their first names only. Plus, being on first name basis with people feels like we know them as friends/family- not having to use their last name actually helps us feel closer to our customers.