That's a very common criticism, but I don't think mathematics would work if you insisted on being 100% explicit all the time.
Clear and short notation, that is just unambiguous enough, is a very important factor, without it books wouldn't just be much longer, I'm not sure we'd even be able to understand it.
This was what Bourbaki's Elements and Whitehead/Russell's Principia Mathematica were about. These books are admired and influential but very few people actually read them. As you might expect, they're too long. They're for giving a different perspective to people who have already achieved the highest levels of sophistication in math.
Clear and short notation, that is just unambiguous enough, is a very important factor, without it books wouldn't just be much longer, I'm not sure we'd even be able to understand it.