Except, that they are not, not at the time they are launched at least. And even if they were, we have a hundred-fold more compute power, with a hundredth of the latency for memory and storage.
Regarding security, it should have negligible effect in most cases. At least, effects should not be perceptible to the human mind.
It really is just a consequence of the way we develop software nowadays. We do not need to optimize programs to make them work at all, so we just do not. We work on new features, and we hire people who can churn new features.
And we decided to optimize for developer time, instead of user time. So, instead of painstakingly developing a Web site, a native application, an Android app, and an iOS app, we just push Web apps everywhere.