Here’s (only part of) why:

Please note that when you add value online, it can take up to 3 days for the value to be available to be loaded onto your card, and you must tag your card to a card reader to load the value. Your card balance will not be updated until you have tagged your card and loaded the value.

3 days? By then I’ll have needed it 3 days ago.

Another gem:

Cable car operators will be equipped with hand held card readers which can verify that a current Monthly Pass exists on your Clipper card. The card readers cannot deduct a cash fare, so customers paying with cash or 10-ride books are not yet able to ride the cable cars using Clipper.

I’ve seen failures on Muni where the stupid reader doesn’t work. No cash? Sorry get off the bus. On Caltrain I’ve seen people with autoload not have their card autoloaded. Operators tend to be understanding (“Oh yea these things kinda suck”), but the current condition is not great.

I realize that this is one step towards the cashless future we see in movies. I just wish it wasn’t going to be a clusterfuck getting there.