Why can't I do this with bitcoin?
Well, today using a third party service I can send Bitcoin to anyone else knowing just that person's mobile phone number or e-mail address (using Coinapult, or Coinbase), or that person's Twitter (using @BTCTip bot), or that person's Reddit username (using BitcoinTip bot), etc.
A problem exists with spoofing though. I can write the message:
Seth is hosting an all-you-can-drink beer meetup for 0.1 BTC. To join, send him payment at: 1PC9aZC4hNX2rmmrt7uHTfYAS3hRbph4UN
And the problem is, .... you did not make this offer, and ... that address isn't even your Bitcoin address. With Simple's P2P system such fraudulent activity is reversible and thus there is less chance that the scammer will profit, so there is very little attempts at such type of fraud. If those transfers of funds were non-reversible their P2P payments system would become a cesspool with spoofed profiles and fake offers, essentially rendering it unusable and giving a black eye to Simple.
Now with the payment protocol coming in Bitcoin-Qt/bitcoind v0.9, verifying such a request as being authentic will be easier.
Additionally, I am cautious about using an app without there being two-factor authentication. But if my mobile is my 2FA device and the app needing the 2FA code is on the same mobile, then 2FA really isn't giving me much additional security. Right now, my only remedy for such a problem is to only hold in that E-wallet app as much money as I'm willing to lose should my device become compromised. (In a similar manner as how I don't generally carry more than a certain amount of cash in my physical wallet in my back pocket either.)
These probably aren't the reasons a Bitcoin-based equivalent to Simple's Money Drop isn't already available, just that improvements to security will help make it so that such an app can actually be built and be useful.