Because it costs $100 year. A self-made SSL certificate is just as secure as one officially recognized by a certificate authority, and it's free. Typically, the only time you want an officially recognized SSL certificate is if you're running an ecommerce site so that visitors don't have a concerned reaction like you are having right now.
An SSL certificate simply encrypts the data between the server and the client so third parties can't see what's being transferred.
Is it unsafe to use a free wi-fi connection that isn't using an HTTPS connection? (That is probably an oxymoron... "safe" and "free wi-fi", but you know what I mean!)
A WiFi connection doesn't use HTTPS--that's reserved for website traffic. It uses WPA, WPA2, or WEP for network encryption. Network encryption prevents third parties from unauthorized access to the network.
But in answer to the question, if you use HTTPS while on a WiFi connection (secured or not), your information is safe. If you use HTTP, everything that is transferred to and from your computer over this protocol is broadcast in the clear (as in, anyone with a packet sniffer can see POST and GET responses).
However, some wireless access points do have a "wireless isolation" feature, which prevents wireless clients from seeing each others' traffic. However, there's not a good way to tell if this feature is enabled without some thorough testing and either multiple wireless devices or cooperation from another wireless user.