The remaining oil needs to be preserved for plastics and other future materials. Burning it is just a waste and harmful besides. A pocket of oil here or there is fine, but it takes years to develop.

They often say that there isn't enough refinery capacity. If not, then why even bother to drill if it can't be refined.

It is all bull; they will do what makes them the most money for the least effort. Who would expect any different? The oil companies evidently concluded that it is better to sell one gallon of gas for $5 than to spend billions to be able sell five gallons for $5.