The resources these technology firms are exploiting in, e.g. Belgium, include user data and network infrastructure - the latter in particular is often very high

The resources these technology firms are exploiting in, e.g. Belgium, include user data and network infrastructure - the latter in particular is often very highly publicly subsidised so if a company is seen as benefitting from it disproportionately then it's unsurprising that people think they should be taxed for taking advantage of it.