The problem is the overly complex tax code that allows entities with deep pockets to find loopholes that don't exist for normal citizens. Solving the problem of big companies not paying the taxes you'd expect them to pay has to involve changing the law.
You can't say "it's the law that needs to be changed" while avoiding doing anything to change the law.
The big tech companies should have tax transparency reports explaining what they do and why it's legal.
The biggest "loophole" is employer payed benefits are untaxed while the same benefits payed out of pocket by the employee are taxed. The political prospects for changing the law here seem bleak.
You can see this played out in the current tax reform debate. Most of the added/eliminated taxes fall in the "eliminating loopholes" category. Here on HN, the proposals have been criticized on the whole.