Taleo is dreadful. Why is it so widely used? Don't they have any competition?

Taleo is enterprise software. With any enterprise software momentum is usually a good starting answer. Replacing not just the software but the support infrastructure and the processes that have evolved around a specific piece of software in an enterprise is costly (time, money, and patience).

The other reason is that the people making the decision to keep it are rarely the people who have to use it. Their decisions are informed my marketing materials, some demos, feedback from people in the company (who also will probably not be heavy users, if at all), and possibly from industry assessments (like Gartner) which are also not based on heavy, day-to-day use. The decisions are also made heavily based on features (no matter how useless to the end user in day-to-day work). This is a general explanation regarding enterprise software. Notice that at no point are candidates involved in the cycle.

There are alternatives (What are the alternatives to Taleo?). At the enterprise level these include Kenexa and Jobvite, and there are additional products geared toward smaller companies and startups. We’ve been working on our own, HireCatalyst to introduce simple job publishing and even social profile application, from LinkedIn to GitHub.