I work at a big Fortune 500 company, we have dozen studios scattered across the globe but we still hire external agencies for a number of internal/external projects.

Reason is simple;

a. they sometimes need a fresh pair of eyes.

b. Might be cheaper and more efficient to hire an external team.

c. External agency might bring in a certain expertise that you may not have in house

d. Internal design team may be preoccupied with on going project.

The reason big companies use design agencies is the same reason they contract out services like legal, auditing, advertising, capital management, janitorial services, catering, architecture, real estate acquisition, etc. etc.

Airbnb, Uber, Slack, etc. almost certainly have in-house legal departments, however they also retain external legal counsel from various specialist law firms. To add some examples to jrtorrents response:

No company does everything well It doesn't make sense to invest in anything that isn't core to your business. Design is an extremely wide discipline. Just because you have a top notch in-house team of product designers working on the core product/service, doesn't mean they would be the best team to put on a marketing site or an advertising campaign when compared to the team at an agency that specializes in that work.

Projects are temporary, in-house employees are not If a firms need for a certain service (ie. marketing design) is cyclical they will often farm this work out to agencies so they don't have have to fire members of their design team after every big project and expose themselves to bad press and lawsuit risks.

External agencies are faster The bigger the company is, the slower it moves. When you've got a big project with a deadline to meet, hiring an external agency is an immediate way to get a competent team on your project without jumping through the slow bureaucracy of your company. For example, say you want to redesign one of your product landing pages, but your team is already at capacity. Instead of taking 12+ months to source, interview, vet a creative director and then source, interview, and vet a team to place underneath that person...the agency could have finished designing that page 10 months ago.

Outside perspective Internal design teams are often averse to change and new ideas after spending years learning about legacy systems, old processes, bureaucracy, etc. They become unable to question the status quo for fear of losing their jobs or Stockholm Syndrome. An outside agency can bring fresh perspective to a project your in-house team is struggling to nail.

Agencies can be cheaper Since agencies often work on a fixed SOW or retainer, their cost is 100% transparent, accountable, tax-deductible and can be negotiated down by your procurement people. You can't walk up to your internal design team and say, "Guys, budget is tight this quarter...will you take 40% less salary on this project?"

Also must realize that at larger companies, design teams are deployed on specific features and/or products. Instead of hiring a new full-time team (time consuming and costly) , it is often cost-efficient to contract out a design agency like Ueno that has a proven track record

They hire external agencies for strong branding and marketing design. The companies that you mentioned have amazing product design teams, but when it comes to creating a one-off marketing site, it's often easier and cheaper to hire an agency for the work.

I think it really boils down to who you know. Having freelanced the majority of my career, your network is everything. People want to work with people they know/trust or have been told to know/trust.

Hustle. They do outstanding work. Good work leads to more good work.