It’s just the luck of the draw. Everyone has different hair patterns. You just happen to have a patchy beard.
I started growing a goatee for the first time about a month and a half ago. It took about three weeks for me to get a decent growth going, where in the meantime I just got comments on how I looked like I needed to shave. My beard hair grows so slowly that I normally only needed to shave every other day to keep my face smooth, and even if I went a week I would just barely have a noticeable 5 o’clock shadow. But given enough time, I now have a normal looking goatee. My advice would be to give it time, and trim it closely to keep it even until the patchy spots fill in.
45 here and can’t grow a decent beard. Goatee/Van Dyke, sure. Cheeks/jawline? Forget it.
Luck of the draw. I wasn’t able to grow a beard until I was 30, even then it wasn’t full. I’d describe it as a medium/light beard as far as density goes, and I have a small spot that never grows any hair. Because of the density I could never grow one of those really long full beards. It would just look to ratty. Moustache comes in full though.
Exactly the opposite for me. I grew a full beard at 16, and got very popular because I could buy beer. Without it, I looked young for my age until I was around 40, so I had one most of the time. It probably started making me look older then, but I’ve been so used to not shaving much I didn’t want to start. Now my beard is nearly all gray, and it makes me look much older. But with my 100th birthday rapidly approaching (only 44 years to go) it doesn’t matter much anymore.
Have you thought about genetics?
I have really odd facial hair, my moustache never gets beyond what you’d see at your local high school freshman class. I have complete but thin beard from the jaw line down but it takes months of growth to look like a beard. I have beard from my side burns down my cheek to meet my jaw line but I have no facial hair on my chin or under my lip and my moustache doesn’t meet my beard. When it comes to body hair I am really hairy, to the point girlfriends plead to be allowed to shave my back and shoulders.
My father could have shaved twice a day if he didn’t want stubble, but my mother said she doesn’t remember seeing her father with facial hair ever.
I feel for you.
I couldn’t grow a respectable mustache and goatee until 30, which coincidentally is how old I am now. I’ve had this Van Dyke for about a year, and I mean to keep it. It fits my face, and I like it.
I’d love me some sideburns, but I think it’s never going to happen.
As advice - I’d say just let everything grow for 2-4 weeks. Shave selectively - you don’t want a nasty neck-beard, so keep it trim. But see what you can get. I tried it about a year ago on a dare/suggestion from a few female friends, and they all tell me it turned out well.
It did (and does) come in a bit patchy for me at first, but when I let it grow I could see the pattern where I could shape/shave it to have a decent look.
I didn’t believe I could actually grow decent facial hair (since I was baby-faced at 23-24 or so) but it turned out at 30 I could manage a respectable visage.
Dad only just within the last ten years or so started being able to manage a goatee. He’s sixty. You’ll get hairier as you get older, but you’ll probably never be rocking it like ZZ Top. I’m in the same boat–best I get is “scruffy-looking.” (Luckily, some people like scoundrels.) I’d love to do ridiculous things like a handlebar or some Flashman “cavalry whiskers,” but it just won’t happen. On the other hand, I’m blessed (IMO) with a relatively hairless body. As my father said, hair won’t grow on steel. 
To notfrommensa, first: its pattern, not patterned; the latter sounds rather, um, phallic. 
Second, I recall from my high school bio/genetics classes that both MPB and being on the hirsute side elsewhere were linked to heightened levels of testosterone; thus, men who were bald up top tended to have full beards and plenty of other body hair. Wikipedia confirms the testosterone connection, but doesn’t necessarily link it to hairiness generally.
Wanna trade beards? If it were up to me I’d get laser hair removal for my face. I have dark brown, almost black beard hair, and even RIGHT after shaving with a BRAND new razor, you can still clearly see my beard. I almost never, ever go more than 2 days without shaving, and if I do, I regret it immensely because shaving on the 3rd day starts to hurt and pull my whiskers instead of just cutting them. I go through 1 razor cartridge per week, can’t take the pain past that. If I were wealthy I’d replace them every 3 or 4 days.
So, count your blessings. Not having much facial hair is not a bad thing.
That may not go away, either. I’m in the same camp at 46. 