Why is getting a PhD considered "financially irresponsible" by some people?
I don’t have experience in industry myself, but know several people who do (and have made the transition from industry to academia and vice versa).

Broadly speaking, getting a CS PhD in the USA with the intent of switching to industry after will result in about $500k in lost wages. This is accounting for about 5 years of PhD salary vs programming salary, raises, bonuses etc. The jobs that necessitate a PhD in industry are few and far between, and are highly competitive. Thus, starting a PhD with industry in mind, and assuming you’ll get that job is, statistically speaking, unrealistic. There are still many good reasons to still do it!


It’s a fun and fulfilling endeavor that offers you a chance to think freely about things you care about for 5 years! 
If you are entrepreneurial then I know some people who start companies based in their PhD work.
If your undergraduate degree isn’t in computer science then your PhD can be treated as a semi vocational degree (though granted a masters will do as well)
If you’re interested in moving to the USA then a PhD offers a path to permanent residence. 
You like research and want to leave the door open to academia. In that case a PhD is a must.