Step expression must be constant?

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Why does the step part of a for/next loop has to be constant? I find this seriously annoying, because is means that i have to rewrite something simple as:
For local x = 0 to width step gridsize

to:
Local x
While x < width
    x :+ gridsize
Wend

It might be a small detail, but I really don't see any reason for the step part to be constant...

Also the help documents say:
Keyword To Followed by a constant which is used to calculate when to exit a For..Next loop.
But the To keyword, luckily, doesn't need to be followed by a constant.