This is more like the what causes these apps to be so large, but when you ask why then you see that they simply do not care about the amount of space they take on users’ devices. There’s no obvious effort to reduce app size with modular design, it simply feels like they don’t care.
As a Google engineer, I believe it is largely accurate to say that we "don't care", or at least "not caring" is the emergent behavior at Google. There are many things we don't care about at Google that I think would shock many engineers who have a healthy amount of pride in craftsmanship. It's hard for me to precisely describe why we "don't care", but I will try anyways.
As the parent commenter has pointed out, pulling in client code can be very large. If the backend team owns the client code, they may not be properly incentivized to improve the product of the clients, sadly. And calling it the backend "team" might be overly simplistic. There may be additional layers to the the client code owned by other teams, such as different protocol implementation and definitions of those protocols, etc. Pushing for change can be viewed negatively, since it could leave a poor impression. E.g. if you improve someone else's code, that could make them look bad, and that would have negative consequences for you as you have violated (a variation of) Greene's first law of power: never outshine the master.
Since the code and the organization are so convoluted and complicated, it's a lose-lose proposition. If you mess up your optimization, you will get blamed. Even if you succeed, you may have reduced someone's reputation of someone you can't even identify in the bureaucracy, and thus have made an enemy of someone you can't even identify.
The Google of 2026 is a very different Google of 2006. In 2006, everyone who left Google had only praises to sing about the employer they were leaving! It is very telling when the Google of 2026 has had years of highly reputable engineers voluntarily leaving (even before the layoffs) who are so bold as to openly criticize Google in the social environment that we're in. Openly criticizing Google requires great personal fortitude, since being a critic only burns bridges and reduces your career opportunities. That is to say nothing about the criticisms that never get published outside of Google.
All large organizations are political. Some employees choose to ignore the office politics, but that choice might find their management not ensuring they survive the next round of layoffs.
I think a simpler explanation the some of the others is.. why care? Phones these days, even cheaper ones, have oodles of GB available. They're not losing customers from the size. And I don't think making it smaller is going to draw in new gmail/workspace customers. So why spend time on it when there are tons of new features or active bugs that could be fixed instead?
This is more like the what causes these apps to be so large, but when you ask why then you see that they simply do not care about the amount of space they take on users’ devices. There’s no obvious effort to reduce app size with modular design, it simply feels like they don’t care.