So I find a new spell scroll and I want to cast it. I need to put it in my book right? What do I do with the spell that WAS in the book? If I have to find it again in order to add it back to my book when I get more space in it, then I have to put the old spell on a scroll and save it for later transcribing back to my book. If spell finding keeps me with generally more spells than I can put in my book, I’m always going to be finagling them in and out in an attempt to have the right ones in there.
This basically causes exactly what you are trying to avoid: I am now trying to out maneuver the DM so that I have the “right” spells in my spellbook for any given week. If, because I chose poorly or because the DM is a jerk, the spell I’ve removed is all of a sudden super useful then I’ve wasted a lot of time and resources for no reason.
If, instead, I could write as many books as I wanted (or even read a spell off a scroll) then I have to balance the utility of being able to cast spells as a mage with the effort of making sure I have the right book/scroll/staff/dagger/implement/etc in my hand, an effort that makes me waste turns as I try to be more versatile and less focused.
And awesomely, as I get better at being a mage I have to do less of that, because I learn to put more spells on the same amount of space. As long as the number of available spells is larger than the number of spells I have in my book, it’s always a battle of balance though.