0.5% is roughly the market price for an otherwise well-compensated engineering role at a company with low financing risk.
Factors that increase equity:
* Founder incompetence, poor negotiating skill, impatience.
* Financing risk (near end-of-runway with no term sheets, &c).
* Sometimes, lack of revenue (at shoot-the-moon startups, revenue isn't expected early on and isn't a factor).
* Below-market salary.
* Non-substitutable technical expertise. Rare. Rails devs may earn a premium relative to the market, but there's still a market for them. SEO, on the other hand, is hard to acquire, because people who can actually do it can almost always earn more freelancing.
* Ownership of relevant core IP; ie, if you came up with the idea behind the company's product, or own an idea that would be key to improving it.
* Frothy talent markets (but, see "impatience" above). If you need someone RIGHT NOW and LOCAL in the NYC startup market, you may pay a premium.
Note that none of these factors are "the primary technical person in a code-based business". That ain't got nothing to do with anything. Yes, that person may be key in proving the business ahead of revenue or funding. But when you got funding and sharply reduced financing risk, you got yourself to a place where you don't have to pay the lead tech person a cofounder's equity grant.