If you spend enough time on the map website you'll see that an all-biome starter island is not very common. (Actually, I've never seen and all-biome start because mistlands are never close to spawn, and far north and ashlands are always at the top and bottom of a map. I've never seen a continent span that far. All currently playable biomes at start is possible though.) Some people reroll over and over to get one like that. Most common is just meadows and black forest, and maybe a small bit of mountain.

Randomization means that there's a decent chance you could end up on a seed that you don't like. If you really hate the one you were dealt, then pack your bags and migrate to a different one.