Two main contributors to this in my experience, and both suggest this is unlikely to change for the foreseeable future.

Part of it is cultural, in that there is still a widespread perception that software engineers are fungible cogs and that no one could possibly be worth that much more than the average technical worker in any other industry -- they value the contribution like industrial labor. I do a lot of work in Europe and many business people find it inexplicable that any software engineer could be worth a senior manager's wage.

The second part is that while wages are relatively low, the cost of employment is astonishingly high in many European countries. Consequently, the fraction of employment cost that gets delivered to the worker as wages is much smaller, all other things being equal. From an employer's perspective, the employment costs are much closer than the paycheck suggests. The company only cares about employment costs, not wages per se. This is unlikely to change anytime soon because it is a manifestation of the social labor ethos common in Europe.