I learned a CMS framework like Joomla for the same reason I learned how to use InDesign, Photoshop, Illustrator, Filemaker, etc. Because it is -- for me -- yet another piece of software that once mastered, allows me to do my work for clients without having to constantly crack open the hood to play and tweak.
Besides, I could hire a developer to build something for me, but who's to say that what he/she builds is any more secure than Joomla? Have you ever had to hire someone to do such work? I can tell you that in my experience it's often a hit-or-miss proposition. You're lucky if the developer responds to you in a timely fashion if you have questions or concerns, and what happens if the whole thing somehow breaks and your developer is nowhere to be found? Sorry, your options for me don't really paint a particularly rosy scenario.
I don't want to code. For me, and thousands of other designers, that's a completely different career direction where I doubt I'd get a lot of personal satisfaction.
The Joomla framework allows me to deploy sites for clients without the need for additional developer help. So far at my current position, we've deployed about a dozen Joomla sites and so far we've been pretty satisfied as have the clients. Yes, problems crop up here and there, but it's nothing I can't handle myself.
No, Joomla is far from perfect, and yes, I suppose some developers who sniff at the idea of using an out-of-the-can software solution have issues with it, but it serves a purpose for people like me and given the alternatives, I'm extremely grateful for its existence.