The things that you're stating as facts are totally opinionated. For starters, metal is HUGELY diverse. Sure, there's some metal with nonsensical lyrics about killing and with screaming rough vocals instead of singing, but you can find plenty of those things in hip hop as well. On the other end of things, there's also plenty of metal that has very meaningful lyrics, melodic and musically complex instrumentation, and sweet melodic vocals. It all depends on the style that you're referring to.
Now, metal isn't ordinarily my cup of tea. For the most part, I don't go out of my way to listen it, save for a few bands, but I at least can appreciate the merit that it has, much in the same way that I appreciate the merit of hip hop and techno.
Meaningful lyrics are subjective and cater to specific audiences. A lot of hip hop caters to lower class communities because it speaks of hardship and street life. It's something they can relate to. Now, I don't personally relate to that, nor do I relate to a lot of metal, but some people do. Much in the same way that I love listening to folk music from the 60s because of the message in the songs, but plenty of people would simply label it as "hippie music".
You're entitled to your opinion, just be aware of what each genre of music brings to the table, and realize that if the name is known, then it's meaningful to someone.
Edit: At the meathead above me, try to be less of an ignorant metal-head. You can't claim that metal's good and then dismiss other genres.
Rap = Cultural. The lyrics speak to certain people, and those are the ones that like it. The style is inspired by poetry, and if you can't understand it, you're not listening.
Techno = Instrumental. Of course the lyrics aren't as meaningful in techno. Techno is about feeling and state of mind. Not to mention that, but DJing is something that's incredibly under-rated and requires a huge amount of talent to adequately create and mix.
Jazz = Soul music. The lyrics speak to a generation that you're not part of, but was hugely relevant at the time. What you're referring to as "can't understand" is likely in reference to scat singing, which is an intentional instrumentation of the voice. You can't understand it because they intentionally want to appear to be non-vocals. I myself am a jazz singer, and I can tell you that there's nothing difficult to understand about Jazz.
Country = Heart. I don't like country, I'll admit, but I can appreciate it. It's soothing and the majority of country songs talk about themes that most people can relate to. Heartbreak, leaving home, finding love, they're real themes. To suggest that all country is about the redneck themes you suggested is pure ignorance.
Many people will say that rock lacks meaning because the majority of rock songs do appear to be nonsensical, but it's what people like, and it DOES speak to someone.