Direct Flights flights from Lisbon(LIS) to Sevilla(SVQ) are not that rare.
The current ana.pt timetable(January 2016) for the route shows:
Two flights daily Sunday to Thursday (morning/evening)
Three flights daily on Friday
One flight daily on Saturday (SVQ/LIS)
One flight daily on Sunday (LIS/SVQ)
48 seater ATR 42-600 turbo-props operated by Portugalia on behalf of the parent company, TAP-Air Portugal. (www.flytap.com)
This represents over 600 seats per week in each direction.
Obviously, if one cannot wait, then a car may be quicker and one gets to see a bit of the countryside.
I suspect that there is additional capacity on the route, particularly, low-cost.