A quick calculation shows a fastest journey time by car on the A380/M5 is about 14 - 15 minutes, 17 miles at 70 mph, add on the time to get from the A380 from Newton Abbot and you are getting near to the journey time by train as the train can actually reach Exeter in about 18 minutes. If you were going to drive in Exeter then you'd lose any advantage due to the traffic. The journey planners most likely add on some time also for the road journey due to the possibility of congestion. The car and train journey times between Newton Abbot and Exeter are fairly close and this is not the reason why the trains from Plymouth are not very busy.
A380 route is 12.2 miles from my old postcode to the m5 junction 30. I lived TQ12 3EH 12.2 miles at 70mph is around 10 1/2 mins. Baring in mind their at several speed cameras along the A380, so no opportunity to exceed speed limit. I lived near to the A380 so would be little traffic between my house and the A380, as it routed along a little used road towards the A380. There was rarely traffic jams on the A380 Exeter bound, unless using between Torquay and Penn inn roundabout, which is required for locals knowing the alternative route via Kingsteignton to the A380 avoiding all the traffic and cutting through an industrial estate and sticking at 70 was never an issue once on the A380.
The railway in Devon and Cornwall makes no sense if you want to get somewhere quickly and efficiently. There is at present no case to dispute this fact sadly.
A good example to use is from where i live now, Penryn, I live 100 yards from the station so no time used in getting into the station at this end. The timings of the train from Penryn to Exeter St Davids is for the quickest timings 2hrs 37 looking at a weekday timetable. The average journey time is 2hrs 54. To drive from Penryn into Exeter City Centre, is on a normal day a maximum of 2 hours. so pretty much an extra hour to go by train.
Mostly west of Exeter like it or not, the main line speeds is no more than those of a branch line. The main line is little more than a glorified branch line sadly. Until the railway is diverted to pretty much run parallel to the A38 Exeter to Plymouth, the train will never compete with journey times by car!