Because those tools are well-known and they allow people to get the result they want. They also can be used for other things and are not specific to a particula

Because those tools are well-known and they allow people to get the result they want. They also can be used for other things and are not specific to a particular kind of drawing. Tikz also allows a tight integration to LaTeX documents that no external tools achieve so easily. The drawing inherits colors and fonts, and bits can be reused across the document and can be shared with other documents too. High learning curve but long term efficiency with quality results. Tikz is not perfect but it has the great characteristic of existing.