Zone meshes FLAC3D

Why FLAC3D can not create a zone with quad structured meshes by using commands? I mean that can avoid infinite small meshes in some cases, like at the centre of a cylinder.

Following the logic that Flac3D uses to create a primitive shape, from a point to another with a number of meshes indicated in the size command, I don’t think it’ll be possible to avoid these small meshes.
You can avoid them using Griddle or other meshers.

In addition to the primitive logic (ZONE CREATE) you can use the Extruder and Building Blocks (or both together) to generate hex structured meshes – both of which are interactive tools that are supported by data file commands that could be sent independently.
A cylinder with a square interior layout that prevents either degenerate faces on the surface or wedge concentrations at the center is something that is supported directly in the Extruder, I believe.

Lastly there is the ZONE GENERATE FROM-GEOMETRY command, which allows generation of unstructured hex-dominant meshes from a closed surface geometry. The plan is that future versions of FLAC3D will expand on this and add interactive support.