Just demand/supply I guess. Semi-conductor companies need just a few (even if extremely talented) engineers compared to software. The industry is also very concentrated and not growing that so not that much competition for employees.
I wonder if it's because of the nature of the companies involved. The semiconductor one is actually producing something tangible with huge costs associated with that. Most of the "tech" companies with the crazy salaries are just in the business of seeking rents from an entire market by collecting royalties or subscriptions for automated things, so they have way more money per person to give away.