There is a growing trend for style guides to use typefaces from no-cost or open libraries. This isn't limited to small companies either, I can think of a handful of industry heavyweights in tech, food and medicine who each have taken up this approach. This forms the first reason not to dupe fonts, because there's plenty of good ones already available for no cost.
I've found that companies that do this have an easier time of staying on brand since it's so simple to obtain the necessary fonts.
As for seeking out deliberate copies of popular fonts, there's a few reasons why an agency wouldn't want to propose this:
1. It's easier to sell in a well loved font, with a proven character from a foundry with heritage. Imagine talking to a board of suits, they're often looking for the safe option.
2. It's kind of a dick move for an agency to be ripping off the foundries, especially small modern ones. Creatives do enjoy what typographers make and can see the love put into their craft.
3. Typographers are not faceless. Modern font designers are still alive and active on socials, you might personally know who you are ripping off.
4. There's a bit of hypocrisy in hating on frankenfonts such as Arial, while deliberately seeking out dupes.
5. It's trivial to open a font in a font editor and resave it with different specs, if a creative wants to be sneaky they can just do this themselves anyway.
6. Adobe have also thrown their hat into this ring by offering a range of popular fonts available for use for no extra cost with their creative cloud subscription. It's not needed to seek out a dupe-font, when the real thing is available with a few clicks.