Why no numeral is needed as a prefix in the naming of butanal and butanone?

No numeral is needed in the naming of butanal and butanone because, in each case, only one number is possible.

The structural formula of butanal is

We don't need to number the aldehyde group, because it cannot be anywhere else except C-1.

If the C=O were on the left hand end, that would become C-1.

If the C=O group were in the middle, as in CH₃CH₂COCH₃, the compound would not be an aldehyde.

The structural formula for butanone is

The C=O carbon must receive the lowest possible number (C-2).

We could write the formula as

But the C=O group is still at C-2.

And there is no butan-1-one. That would be butanal.

So we don't need a number for butanone, either.