Nightgowns For 6 months + Why Wouldn't They Be More Common?

Searching stores in person and online and I can't find more than a handful of nightgowns for 3-6 mo. Why?

In the night when I'm changing a diaper the last thing me or my son wants are buttons or zippers. He just wants to feed and the longer a diaper changing takes the more awake he becomes. I'd think TONS of other parents feel the same way. So why aren't there easy to find nightgowns for older than 6 months old?

I'm going to try and sew a few and am fully capable of doing so, but finding time to sew is hard with a baby to take care of.