Why do so many PhDs choose to join academia instead of industry given the pay in academia is considerably lower and the workload is much heavier?

In CS field, an entry-level software engineer in top-tier tech companies (e.g. Google, Facebook, etc.) could earn as much as a 20-year experience professor....Why do so many PhDs still choose to be a professor while they have the choice to go to the industry?

First, your assumption is wrong. Most PhDs end up in industry. I don't have any source but I think this is from 90 - 99%.

If you limit your question to the 1% - 10% that become professors, they are all very successful, i.e. they have plenty of papers, promising research direction, strong network etc etc. And there is a reason for their success: they have passion, and when you have passion, money is likely not the most important thing in life.

Except for machine learning, in most areas in CS, you need to stay in academia to do research. And there are many benefits that you can only have when working in academia.

• You take credit for what you have done. Products in industry are developed by a large team, and nobody can take full credit for it. But researchers can take full credit for what they do in their papers.
• Reputation: you are invited to give talk, become program committee members, etc etc, and everybody will know you. I would be excited to meet an author whose paper I have read. I'm not excited at all to meet a Google employee. (I'm living in Mountain View, a small city with 80,000 residents, but more than 20,000 Google employees)
• Do interesting jobs. You always work on new things in research, while the majority of tasks of a software engineer are maintenance, fix bugs etc.

(I'm a software engineer if you are curious)

I've worked in industry science, and in academic science (as a PhD for both - I'm back in academia now).

Industry Science

• The problems you're working on are tangible and usually very interesting
• You have resources available
• There are good things about a real HR department (never had a missed paycheck in industry science; never really worked more than 40 hours a week)
• Most of your coworkers, even the dumb ones, are pleasant enough at work.
• You will spend at least half your work week in meetings. Most of which will be valueless
• Corporate middle management is usually staffed by people who are arrogant and self-serving enough to escape actual work, but too dumb to really do anything important. These people will usually supervise you. Daily.

Academic Science

• The problems you're working on may be interesting or they might not be. Academic freedom is a misnomer - if you want to be successful, you're still as constrained in what you can do as industry science (IMO)

• You are more resource limited, but usually have free or cheap labor available

• There are good things about a fake HR department (you can finish all your annual trainings in 20 minutes on a computer)

• You may spend a lot of time on administration, depending on your role

• You may spend a lot of time teaching, but it's generally somewhat rewarding unless you're teaching intro stats to 600 people who would rather be high

• Most of your coworkers, even the unpleasant ones, are smart and provide an intellectual environment that's usually positive

• You usually don't report day-to-day to a idiot chimpanzee

• You can make fun of corporate-speak without having a meeting about being a team player.

So I mean... there's good and bad on both sides. The pay gap exists but isn't as big as people think in most fields (when I left my last industry job for this academic job, the salary was pretty much the same). Just do what makes you not want to stab people every morning.

As far as I see it, the main advantage that academia has over industry is freedom. In industry, you generally work on what your employer tells you to work on. As a professor, even at the entry level (assistant professor), you have quite a bit of leeway to work on what interests you, with the constraint that you have to find an agency that will give you money for at least some of it. Many postdocs and even some graduate students have the ability to come up with their own project ideas and pursue them, as long as they are somewhat consistent with their mentors' funding streams. The constraint of being able to find funding is a big one, but I still think that I have much more freedom than my friends in industry. As a professor, I only talk to my boss about what I'm doing once or twice per year. In academia, usually your boss is happy as long as you bring in grants.

Another reason to stay in academia is inertia. Once you've spent 5 years getting a PhD, you know a lot about what a career in academia looks like and have resources to help you move forward. Finding how your skills might fit in industry is less obvious and you might not know how to start.