It’s a fair challenge. Whilst the distance is greater, the rail journey Plymouth - Tavistock would be roughly the same time as Exeter St David’s - Okehampton, whilst the Tavistock road journey would be longer. However it’s not the benefits side of the equation that is the issue, it’s the other side….
The three main differences are:
1) Okehampton was an existing and ‘active’ railway, and therefore needed no primary consent to reopen. Tavistock is not, which costs time (lots of it), and time=money.
2) Okehampton was around £40m (+ £1). Tavistock would be 3-4 times that, with similar forecast passenger numbers.
3) Okehampton fell very fortunately into a spot where there was industry political imperative to show it could do something quickly, and political political imperative to show delivery of policy. Because of (1) above it had the opportunity to deliver quickly, and was therefore chosen. Tavistock does not have that opportunity.