There are many reasons. Firstly, structural engineering mostly generate revenue in INR instead of USD as software companies generate. The growth of structural engineer is slow because 2-3 years of experienced person cannot become a project manager in any company because if the structural fails, it may become a national news. Further, our market is highly informal and also there are challenges for cash flow. If you need a good salary, I suggest you work in Marine field, bridge or any other specialized field.