TOareaFan
Superstar
It's for the same reason that VISA cards expire after a few years. It allows them to replace the card with an updated one using newer technology if appropriate.
I can understand the general sentiment, but as above, it's a $1.20 a year. I don't think I'll be worrying about that.
NOTE to ANYONE currently riding GO; you will be able to get a "free" Presto Card when Presto comes to your station. The deal is that you turn in a 10-ride or monthly pass and you get a Presto card with equivalent value loaded onto it back in return, without having the pay the $6 issuance fee. They do give you your cancelled pass back so that you can use it for tax purposes if appropriate.
Details are here: http://www.gotransit.com/public/en/fares/prestofaresMay1.aspx
Visa, and other cards, expire more for credit review purposes. I may have qualified for that card 3 years ago but I may not 3 years from now..sorta thing. Credit cards can (and are) replaced mid-term if new cards with significant new features are released. Got a new one yesterday, as an example.