You could always try asking him yourself, he seems pretty responsive to people on twitter, and here [1] he answered a somewhat related question.
[1] https://twitter.com/justinamash/status/1339970633422221312
It's not just that.
After they've left political office, politicians who've advanced the interests of some companies while they were in office often get extremely well paying jobs at those very companies.
High level corporate executives will also regularly go in to politics (Trump being Exhibit A, but there are plenty of others at lower levels).
This revolving door of corporation to political office and back is an open secret in government.
Yet the impression among the general population is that corporations and the government are clearly separate entities. They're not.
I get the sentiment, believe me. But "being principled" is a matter of degree and perspective.
Clearly, there is a gap between what a particular individual wants of their elected representatives ("reps") versus what they get. Why?
Sure, there are charitable explanations. First, reps have to balance competing interests. Second, some reps may make 'principled' compromises in order to prioritize certain values over others.
There are less charitable explanations, too. First, many reps seek power as an ends unto itself, but doing so pulls them into a system where 'principled' decisions (e.g. ones directly for the benefit of voters) become harder. Second, some reps have questionable ethics and are willing to take money at the expense of their proper responsibilities. And so on.
All of interpretations are true, to some degree.
If people in a (more or less) democratic system want to change it from within, it is not effective to simply hope for reps to behave more ethically. We have to: (1) adjust the rules; (2) change our expectations; and (3) get involved: run for office, volunteer for campaigns, give money and resources, and vote. (Tell me if I've left anything out!)
> Please don't comment about the voting on comments. It never does any good, and it makes boring reading.
Of course - this is his last term, he did not run for reelection. I hope he does something meaningful next.