It's the same as CMS/Todos/PM/Crm software, most of small companies create their own... and after wasted year on broken, non tested software they look for something commercial. $5 is a good price for simple software.
Why write your own (if you meant "from scratch") when there are bunch of FLOSS software projects that either already suffice user needs or beg for contributions? ;)
If you were a freelance programmer, you would be much better off taking the time you would spend building a time-tracker from scratch and instead spend it on billable client work at a much higher rate than $5 per month.