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Why don't they just use Oyster card for this?

I'm sure the data from that is much more fine grained.

Update: Ah got it. With Oyster they can only register the entry and exit points and not the route taken.

When connecting between different tube lines, much of the time users remain inside the ticketed area of the station, so only tap their Oyster/contactless card when they pass through the entrance/exit barriers at the very start and end of the journey. (Though there are a few stations where there is not a direct tunnel between lines, and travellers are required to connect via a separate street-level entrance, where they would tap their card mid-journey.)

In cases where there are multiple routes to complete a trip (eg remaining on one line vs making multiple connections for a faster journey) it was therefore previously not possible to determine what percentage of people chose which route.

The Oyster card will not tell you what route a commuter took, only the start and end point.

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