Because your female lalafel doesnt really have 6-pack abs, it's just the design of the gear, you know similar to how roman and greek armor were sometimes formed to resemble the body? But if you want to get technical about it, tanking and other physical classes would realistically chisel one's physique to be more muscular... and females would not be exempt from this fact. But all in all it's just part of the design of the armor.

And lazy? You do realize quite a bit of work goes into creating gear for an MMO, especially games that have multiple sizes of PCs including adjustments for height. It's actually quite ridiculous to ask an art team to create different sets of armor also based on gender for every class/race combo.