Obviously there are agents that are like this, but imagine this scenario:

-- You want to buy your first house, I sell you a house. -- You have a couple kids and want a bigger house. I help you sell your house and buy a new house. -- Your stock is doing well and you want a vacation house, I help you buy one. -- You want an investment property to send the kids to college and/or pad your retirement fund. I help you buy one. -- You are ready to retire and move someplace else, I help you sell your house and I refer you to an agent in the place you want to buy.

Thats's seven deals I can potentially help you with, if I take care of your needs and earn your trust and business. Compare to what happens if I don't do right by you and you go to a different agent - I get one deal, no referrals to your friends, no nice words from you I can put on my website, etc. So sure, there are agents that are purely transactional, but those agents are doing it all wrong - for you the client and for themselves too.