You are correct, if everyone had these plugins then everyone would look the same.
However, let me share a couple of quotes:
From the FAQ:
Some people have suggested we include ad-blocking software or
anti-tracking software with Tor Browser. Right now, we do not think
that's such a good idea. Tor Browser aims to provide sufficient
privacy that additional add-ons to stop ads and trackers are not
necessary. Using add-ons like these may cause some sites to break,
which we don't want to do. Additionally, maintaining a list of "bad"
sites that should be black-listed provides another opportunity to
uniquely fingerprint users.
From the The Design and Implementation of the Tor Browser document:
No filters Site-specific or filter-based addons such as AdBlock Plus, Request Policy, Ghostery, Priv3, and Sharemenot are to be
avoided. We believe that these addons do not add any real privacy to a
proper implementation of the above privacy requirements, and that
development efforts should be focused on general solutions that
prevent tracking by all third parties, rather than a list of specific
URLs or hosts.
Implementing filter-based blocking directly into the browser, such as
done with Firefox' Tracking Protection, does not alleviate the
concerns mentioned in the previous paragraph. There is still just a
list containing specific URLs and hosts which, in this case, are
assembled by Disconnect and adapted by Mozilla.
Trying to resort to filter methods based on machine learning does not
solve the problem either: they don't provide a general solution to the
tracking problem as they are working probabilistically. Even with a
precision rate at 99% and a false positive rate at 0.1% trackers would
be missed and sites would be wrongly blocked.
Filter-based solutions in general can also introduce strange breakage
and cause usability nightmares. For instance, there is a trend to
observe that websites start detecting filer extensions and block
access to content on them. Coping with this fallout easily leads to
just whitelisting the affected domains, hoping that this helps,
defeating the purpose of the filter in the first place. Filters will
also fail to do their job if an adversary simply registers a new
domain or creates a new URL path. Worse still, the unique filter sets
that each user creates or installs will provide a wealth of
As a general matter, we are also generally opposed to shipping an
always-on Ad blocker with Tor Browser. We feel that this would damage
our credibility in terms of demonstrating that we are providing
privacy through a sound design alone, as well as damage the acceptance
of Tor users by sites that support themselves through advertising
Users are free to install these addons if they wish, but doing so is
not recommended, as it will alter the browser request fingerprint.