Dirt jumps with wood
Properly treated timber will last decades in the ground. Wood treatments are generally rated for there preservation ability. Different treatments are used for protection from insect and moisture damage, and the timber is treated to suit the local conditions. Most countries have a set of standards - e.g. American Wood Protection Association for USA.
Some timber will last several years in ground with no preservative, and a lower level preservative treatment might last long enough for your purpose
As far as longevity - the problem really comes down to safety - who will know when the timber has degraded, who will check, and what damage will result form a collapse?.   Also you need to consider the construction methods - building something to last a few days is very different to building something to last a few years - do you have the wood working skills to be sure you will built the structure strong enough to last years of jumps.
While you could build a structure in timber, cost factors into it. You not only need timber, you need the fasteners and time and tools to build the structure. Dirt is cheap, and for jumps versatile and easy to change profile - most likely it will be much cheaper to get a truck to drop off the dirt you need to build the jumps,  but do cost out having the dirt taken way when you no longer want the jumps.  It your that way inclined, its also a decent an excuse to hire an excavator and have some fun...