Because the default should be to block tracking. We just shouldn't allow it. Imagine someone telling you a popup blocker (something even IE8 had standard) should be acquire separately.
I absolutely agree that browser vendors have huge conflicts of interest, and we need to address those by forcibly separating browsers from companies inherently in conflict with privacy.
But it makes you wonder, if other browser vendors' interests do conflict with ad blocking, why would they ship with uBlock Origin? They would want to control what most people do by controlling the defaults. The vast majority of users never change the default settings.
There's not a mission-level conflict-of-interest, but there's a practical one. Until they can somehow be funded directly by users, Firefox will have this tension of "don't piss off Google or other referral partners, even if that means going against the users' interests". They won't serve the users well by losing all their funding and then dying.
This situation is why Firefox and Mozilla aren't just everything they could be in terms of completely aligned with users.
If Firefox was solely aligned with protecting their users, the default search would be DuckDuckGo, not Google, and it would fully block Google Analytics, Fonts, and AdSense right out of the box.
Which is why they have things like telemetry, backdoors like normandy, tracking via safebrowsing, unblockable tracking via google analytics in about:addons and any mozilla pages, a great track record with pocket and the mr robot addon, tracking-related bugs which have gone ignored for years, and richly paid executives, right?