Secondly, most of the willows in front of the tomb are not planted artificially, but basically grow naturally. Although willows are easy to survive, they need at least suitable soil conditions to grow. The graves of the rich are basically compacted with concrete, so it is impossible to grow trees naturally outside the graves.
For example, trees planted with pine and cypress were often tombs of princes and princes in ancient times. There must be a lot of gold, silver and jewels in the grave. Ordinary people can't plant trees in the grave. The growth of willow means that the foundation of the grave has not been treated, so it can't be the grave of the rich. Stealing this grave may be thankless, so grave robbers will not touch it.
Finally, grave robbers attach great importance to their fortune. For example, grave robbers often act together with their uncles for good luck, which is the homonym of "saving" and "living". Grave robbers are superstitious about this, and the homonym of willow is "stay". The ancients often used willow branches to retain friends and guests, but putting them here is not a symbol of good luck. Grave robbers don't want to be left in the grave, so tombs with willows often don't touch them.