It's not true that you can learn everything online. Online, you may be able to fulfill a lot of the knowledge level learning from Bloom's taxonomy, you may be able to fulfill a subset of the comprehension level learning, but online learning is very poor (for most professions) for the higher levels, such as application and synthesis.
We go to university/college to prepare us for a career. A degree documents learning. The primary value of a degree is that it labels you as having some basic set of skills and knowledge. It labels you as being able to succeed within a system that is not under your control.
Imagine you are responsible for hiring individuals for a large company. You publicize that you have an opening for an entry-level position in a skilled profession. You get hundreds of applications. How do you filter those down to people that may actually be qualified? You set prerequisite requirements. The most common of which is a degree from an accredited educational institution, in a discipline related to the job position. Employers have to trust that the educational institution would not have given you the diploma if you didn't earn it. Without that diploma, how would an employer be able to have any confidence that you know what you need to know? That you have the skills you need to have?
I mentioned, above, that a degree also indicates that you have some ability to succeed within a system that is not under your control. Without going to school, you may indeed have spent four years in your parents' basement studying and learning via online sources of information, but that doesn't mean you will fit into the real-world work environment. The real-world work environment involves having schedules imposed upon you. It involved being given orders. It involves participating in teamwork. It involves accomplishing goals by cooperating with people you don't like. It involves communicating via oral and written methods. All of that is also part of the educational experience. By completing a degree, we show that we have some toughness and that we don't walk away from challenges. That is what a hiring official really wants to see in the people they select.
I talk to kids about becoming engineers. I tell them that they probably won't use a lot of the material from their textbooks, on the job. I did hundreds of Taylor series in college, but I've never had to do one as a professional engineer. I've never had to write a line of Fortran since college. I can't remember Young's modulus of any of the materials I memorized in college. Most of the knowledge I needed for my job wasn't taught in school, it was learned on the job.
But, performing all of that engineering problem solving made me a problem solver. Writing all of those lab reports and presenting my results made me a more effective communicator. Showing up, day after day, to classes I hated and not fulfilling my daydreams of making lousy or nasty professors eat the chalkboard erasers made me more disciplined. All of that made me a better candidate for my job. A good university education teaches students real-world coping skills using the context and vocabulary of their desired field. Studying online isn't very helpful in developing those skills.
Depending on the field one wants to pursue, an educational institution can also be important because it can provide resources that people can't provide for themselves. It's not easy to get a cadaver to dissect at home. Electron microscopes aren't cheap. One usually can't make Kevlar at home. And no matter how much Googling one does, nothing beats being able to sit face to face with an expert in a field and talk to them - ask them questions and hear their war stories.
(All of that said, I do believe the extreme increases in the price of education have made a degree cost more than it is worth).
