It may be a waste of space, yes, but space is cheap.
Also, it is extremely valuable to have these tools on every browser I happen to use. I can see what's wrong with my website--or anybody's website--at any computer. If you go to some page and something doesn't work, I can just open the developer tools on your browser and try to sort it out.
So while most people don't need these tools most of the time, they can still be useful once in a while and have a negligible cost.
Some stats:
~350 million users[1]
~17Mb download[2]
Lets assume a full 25% is dev tools (I think its likely to be much less), so 4.25Mb per user. That's 1418Tb, or just under 500 3Tb disks. Considering that's spread across 350 million users, I think that's not too much at all, and its probably much less.
Also, it is extremely valuable to have these tools on every browser I happen to use. I can see what's wrong with my website--or anybody's website--at any computer. If you go to some page and something doesn't work, I can just open the developer tools on your browser and try to sort it out.
So while most people don't need these tools most of the time, they can still be useful once in a while and have a negligible cost.