Why? Two main reasons:
(1) Trust: The site is not owned by a trusted party. The owner could just change the URL my shortened URL points to. And I'd look darn silly promoting a link to a different page, which could have horrendous content or my endorsement of the URL could be abused for different purposes.
(2) Reliability: The service is definitely less reliable than an official Google-hosted service. The link might just stop working or might be slow (latency), etc.
Here is the website where signatures can be made: