Because a scroll is a one-use version of a spell. In the same way that writing a prepared spell into a spell book wipes it from the caster's mind, transferring it from a scroll (which is just like a prepared spell slot in written form) wipes it from the scroll.
They are two different things. Note that you also can't cast the spell directly from the spell book as you could with a scroll. The letters on the scroll ARE the magic item (the scroll itself is just a piece of parchment). When that magic is expended (either to cast the spell or allow you to write a version of it into a spell book) the letters disappear... the magic is only usable once. Spells in a spell book on the other hand are NOT magic items. The letters are just ink on a page. No magic to be expended.