Tar survives for decades because many of its heavy organic compounds are slow to evaporate, slow to wash away, resistant to water, and able to remain in pores, cracks, and sheltered surfaces.
Tar can look temporary when it is fresh and sticky, but parts of it are remarkably persistent. Heavy fractions remain after lighter smells and oils have faded. Those fractions can stay locked into wood grain, rope fibres, soil, old seams, and building surfaces for a very long time.
Persistence depends on exposure. A thin coating on a sun-baked fence may weather away much faster than tar trapped inside a joint or preserved under another layer. Archaeological tar residues often survive because they are sheltered from the full force of air, sunlight, and abrasion.
Water resistance also helps. Tar does not dissolve easily in rain, so it is not washed away like a water-based material. Instead, it erodes, oxidises, cracks, abrades, and slowly changes. That slower path allows traces to remain even after the original surface has aged heavily.
Decades of survival do not mean decades of perfect protection. A tar trace can remain long after it has stopped doing useful work. The visible survival of tar and the performance of tar are related, but they are not the same question.