(Aside: what's the currently accepted pronoun there? I was always taught "they" is incorrect grammar, but without devolving this discussion, that seems to be accepted now?)
My son is 13 months old now, and crawls like a maniac, ever since he was roughly 5 months old. Very social, talkative, active, all those good things. Pulls up and stands all the time, can climb in and out of toy vehicles, climbs stairs, all of that. When I hold him while he stands, he is barely putting any weight on me. Even if he's not walking, he should be able to stand on his own. But he doesn't, and he absolutely refuses to do so. He sees cousins and other big kids walking, and isn't afraid to crawl to them and play, but it doesn't inspire him to walk.
We know a lot of other kids that are younger than him, his age, or a few months older, and they are walking. He's stronger and more capable than them, but they're walking and he isn't.
My wife and I comment on this fact all the time, about how it seems that walking has nothing to do with strength, and it's more about readiness. I doubt that my son isn't walking because he's not strong enough, nor is it because he doesn't have a good environment, save for the fact that I'm not pushing him to walk (i.e. I'm enjoying the calm before the storm.)
Readiness for walking is definitely dependent on a number of factors, but I'm willing to bet that genetics is a big part of it too. Apparently I didn't really walk until I was 2 years old.
Babies and walking is a very interesting phenomenon that really lets you see into the working of genetics and environment.