Why doesn't Boolean work properly in Blender?

Such problems are usually due to the fact that the mesh has “defect”.

Incorrected Normal, loose geometry, internal surfaces, areas without thickness as well as overlaps of surfaces, etc. can lead to errors during cutting, but also in simulations, during rendering or 3D printing. In mathematics this means Multifaceted; practice does not conform strictly to the mathematical definition, yet the term “non-manifold geometry” in the field of computer graphics extensive application in this respect.

Most of them have to do with discipline when creating models. Usually an object should be built from a continuous mesh, not individual cubes. A helpful tutorial series in the direction is the following [English]: https://www.youtube.com/playlist?list=PLjEaoINr3zgEL9UjPTLWQhLFAK7wVaRMR

https://www.youtube.com/watch?v=Hf2esGA7vCc

But obsoletely relevant from the concept:

https://youtu.be/t_C447Ikjjg

Before you completely rediscover the model, you can first try these things:

1. Recalculate the normals of the container in processing mode: https://docs.blender.org/manual/en/4.3/modeling/meshes/editing/mesh/normals.html#recalculate
2. The Boolen modifier also has the “Fast” Solver as an option. This is less accurate in geometry without defects, but can also be more error-tolerant.
3. You can recalculate the mesh of the entire container so that a shell is created without defects (here details can be lost): https://docs.blender.org/manual/en/4.3/modeling/modifiers/generate/remesh.html || https://docs.blender.org/manual/en/4.3/modeling/meshes/retopology.html#remeshing