There could be several reasons why your trees and shrubs keep dying, some of the common reasons are:
Planting too close together: Trees and and shrubs require space in their environment not only for their roots but also their canopy to grow. If you plant trees too closely together, they will fight for resources. Planting too closely together can also cause poor airflow which can lead to disease issues.
Poor Soil Conditions: Trees and shrubs require specific soil conditions to thrive. If the soil is not suitable for the type of plant, it may not be able to absorb the necessary nutrients, leading to its death.
Overwatering or Underwatering: Overwatering or underwatering can cause stress to the plant, which can lead to its death. The roots may rot in overwatered soil, while underwatering can cause the plant to become dehydrated.
Pest and Diseases: Trees and shrubs are susceptible to pests and diseases that can damage or kill the plant. Common pests include aphids, spider mites, and scale insects, while diseases like root rot and leaf spot can also be harmful.
Improper Planting: If the tree or shrub is not planted properly, it may not be able to establish itself in its new location. Improper planting can lead to poor root development, which can cause the plant to die.
Climate Conditions: Trees and shrubs require specific climatic conditions to thrive. If the climate is too hot, too cold, or too dry for the plant, it may not be able to survive. Each species of tree and shrub has a specific environment that they are adapted to grow in, this is important to consider when selecting the right tree for the right space. Trees often are selected based on their ability to withstand the extreme cold temperatures, this is called hardiness. You can reference the hardiness zone map here
It is best to consult with a local gardening expert or arborist to determine the exact cause of your tree or shrub's death and how to prevent it from happening in the future.