Any answers people can give will only be pure speculation. Like maybe Square recognized that the dated gameplay of FF1 and 2 wouldn't sell as individual games, or being the first Final Fantasy port they weren't certain what the demand would be or how the marketing would work if they attempted them individually.
Maybe it's the simple fact that those were NES games, whereas by modern standards SNES games can warrant individual releases considering their length and depth in gameplay.
Like I said--pure speculation. If you don't think it's worth buying, then don't. But if they don't release 5, 6, or whatever other games you'd like to see, don't bitch later. You can support their efforts or not, either way you're not going to force Square into acting in a manner you believe is kinder by not spending money.
Anyway, all information so far indicates (in other words, this is pretty much a certainty) that FF5 and FF6 will be released and sold seperately. And that isn't happening until later next year, after people have a chance to get this game. It's safe to assume that a comparable period of time will pass between 5 and 6 too.