How come pairs are slower than solo riders at the Transcontinental Race?
I expect that the three main contributors are:

The solo riders are just stronger. There's more glory in the solo win so, if you are strong enough to, say, come in the top five on your own, it's probably better to do that than come in the top five as a pair.  For example, compare tennis: the really strong players concentrate on singles.

It's easier to look after yourself and know your own limits than try to figure out what your partner's limits are. At this level, the question isn't so much &quot;Can I ride for another hour?&quot; but &quot;How motivated am I to ride an extra hour even though I feel like crap, and can I keep this up to the end?&quot; It must be very hard to figure out exactly where on the scale your partner is.
For a trivial example if this, how often do you end up in the following situation? You and your partner/friend want to go out for a meal. You suggest restaurant A, they suggest restaurant B. You end up saying, &quot;No, no, it's OK, let's go to B&quot; while they're saying, &quot;Really, A is fine. Let's go there.&quot;

Drafting doesn't give you much at the kinds of speeds the riders are doing.
The winning average speed (based on Wikipedia's statement that the race is about 3900km; averaged over moving time) was only about 23km/h (14mph); the leading pair was at about 19km/h (12mph). So, in reality, the solo riders go at their own speed; the pairs go at the speed of the slower member.


Plus, as mentioned in the comments, two bikes will have twice as many mechanicals.
The pairs category actually has less restrictive rules. The 2017 Race Manual says:

In  the  pairs  category  riders  in  the  pair  act  as  a  unit  and  may  share  food,  equipment,
information and resources
between themselves and help each other including riding in each
other's slipstream. No support is to come from outside the pair and resources cannot be
shared  outside  the  pair.    To  all  intents  and  purposes  the  pair  shall  interact  with  others
outside of the pair as if they were a solo rider.

In contrast, non-paired riders aren't allowed to slipstream or share information or resources. The same text has appeared in the manuals since at least 2014 (except for a completely trivial change of wording). I've not been able to find the 2018 manual but I assume nothing has changed, still.