That's an excellent and provocative question. The premise that no one uses desktop computers and that web applications are universally slow on mobile devices is a common, but not entirely accurate, perception. The popularity of JavaScript (JS), in a world dominated by mobile and web applications, can be attributed to several key technical and practical factors.

1. The Premise is Not Entirely Accurate

First, let's address the premise. While mobile usage has surpassed desktop usage for general browsing, the desktop computer remains an indispensable tool for a vast number of tasks.

Professional and Creative Work: Software development, data analysis, graphic design, video editing, and complex content creation are almost exclusively done on powerful desktop machines.

Gaming: High-performance gaming is still a major driver for desktop PCs.

Enterprise and Office Environments: Many businesses and educational institutions rely on desktop computers for daily operations.

For these users, the desktop web is a primary interface, and desktop web applications have become incredibly powerful and feature-rich, often replacing traditional desktop software. JavaScript is the foundational technology that powers this transition.

2. The Rise of Single-Page Applications (SPAs)

The "slow and horrible" web app experience on mobile is often a problem of poor implementation, not an inherent flaw in JavaScript itself. In fact, JavaScript, through modern frameworks, has been key to solving some of these performance issues.

Asynchronous Operations: JavaScript's event-driven, non-blocking nature is perfectly suited for modern web applications. It allows a web page to make network requests (e.g., fetching data from an API) without freezing the entire user interface. This is crucial for creating a smooth, responsive user experience on devices with intermittent or slow network connections.

Single-Page Applications (SPAs): Frameworks like React, Angular, and Vue.js enable the creation of SPAs. In an SPA, the initial page load might be slightly heavier, but subsequent navigation and interactions are extremely fast because the entire page doesn't need to be reloaded. Instead, JavaScript handles dynamic content updates by fetching only the necessary data. This makes the user experience feel native and instant, even on mobile.

3. "JavaScript Everywhere" and the Unified Tech Stack

One of the most powerful arguments for JavaScript's popularity is its unique ability to run on almost any platform.

Universal Language: JavaScript is the only programming language that runs natively in every major web browser. This means a developer can write a single codebase and have it work on Chrome, Firefox, Safari, and Edge without modification, guaranteeing accessibility to billions of users.

Full-Stack Development with Node.js: The introduction of Node.js revolutionized JavaScript. By bringing the Chrome V8 engine to the server, Node.js allowed developers to use JavaScript for back-end development. This creates a unified "full-stack" tech stack, where both the front-end and back-end can be written in the same language. This leads to several technical advantages:

Developer Efficiency: Developers don't need to context-switch between languages, which streamlines the development process.

Code Sharing: Logic can be shared between the server and the browser, reducing code duplication and potential bugs.

Performance: Node.js excels at handling a large number of concurrent connections due to its non-blocking I/O model, making it ideal for real-time applications like chat and streaming services.

4. Massive Ecosystem and Developer Community

Finally, JavaScript has an unparalleled ecosystem and community.

Vast Library and Framework Ecosystem: The sheer number of libraries, frameworks, and tools available for JavaScript is staggering. From specialized data visualization libraries like D3.js to comprehensive testing frameworks like Jest, a developer almost never has to "reinvent the wheel."

Open Source and Community Support: The JS ecosystem is largely driven by a vibrant open-source community. This ensures rapid innovation, continuous improvement, and robust support, making it easier for new developers to learn and contribute.

In conclusion, JavaScript's popularity is not a paradox but a testament to its technical evolution and adaptability. It has matured from a simple scripting language to a foundational technology that powers the modern web on both desktop and mobile, thanks to the rise of sophisticated frameworks, its full-stack capabilities with Node.js, and an unparalleled developer ecosystem. The perception of "slow and horrible" web apps is often a remnant of an older era of web development and not a reflection of what modern JavaScript is capable of.