Web pages are big

Web pages load their own user interface code.

There are two immediate consequences of this.

First, each web page must load its own libraries to implement that interface. That slows the loading of web pages. The libraries include more than code; they include stylesheets, fonts, graphics, images and lots of HTML to tie it all together. The code often includes bits to identify the type of device (desktop web browser, mobile web browser, etc.) and functions to load assets when needed ("lazy loading").

I imagine that lots of this code is duplicated from page to page on a web site, and lots of the functions are similar to corresponding functions on other web sites.

Second, each web page has its own user interface, its own "look and feel", to use a term from the 1980s.

Each web page (or perhaps more accurately, each web site) has its own appearance, and its own conventions.

Even the simple convention of "login and logout links are in the top right corner" is not all that common. Of the dozens of web sites that I frequent, many have the "login" and "logout" links in the top right corner, but many others do not. Some have the links close to the top (but not topmost) and close to the right side (but not rightmost). Some web sites bury the "login" and "logout" links in menus. Some web sites put one of the "login" and "logout" links in a menu, but leave the other on the page. Some web sites put the "login" link in the center of their welcome page. And there are other variations.

Variation in the user interface is not evil, but it is inconsistent and it increases the mental effort to visit different web sites. But what do the owners of each web site care? As long as customers come to their web site (and pay them) then the web site is working, according to the company. The fact that it is not consistent with other web sites is not a problem (for them).

Web sites have to load all of their libraries, which increases overall load time for the site. The companies running the web sites probably care little, as the cost is imposed on their customers. The attitude that many companies take is probably (I say "probably" because I have not spoken to companies about this) is that the user (the customer), if dissatisfied with load time, can purchase a faster computer or a faster internet service. The company feels no obligation to improve the experience for the customer.

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The situation of individual, differing user interfaces is not unique. In the 1980s, prior to Microsoft Windows, PC software had different user interfaces. The word processors of the time (WordPerfect, WordStar, and even Microsoft Word which had a version for MS-DOS) each had their own "look and feel". The spreadsheets of the time (Lotus 1-2-3, Quattro Pro, and Microsoft Multiplan) each had their own user interfaces, different from each other and different from the user interfaces for word processors. Database packages (dBase, R:Base, Clipper, Condor) each had their own... you get the idea.

Windows offered consistency in the user interface. (It also offered graphics, which is what I think really sold Windows over IBM's OS/2, but that's another topic.) With Windows, programs started the same way, appeared the same way, and provided a set of common functions for opening files, printing, copying and pasting data, and more.

Windows arrived at an opportune time. Computers were fairly common, people (and companies) were using them for serious work, and applications had their various user interfaces. Windows offered consistency across applications, and reduced the effort to learn new applications. A spreadsheet was different from a word processor, but at least someone who was familiar with a word processor under Windows could perform basic operations in a spreadsheet under Windows. People, when learning new applications, could focus on those aspects that were unique to the new applications, not the common operations.

The result was that people learned to use computers more rapidly than in the earlier age of MS-DOS. Windows was sold on the reduction of effort (and therefore costs) in using computers.

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Will we see a similar transition for the web? Will someone come along and sell a unified interface for web apps, advertising a lower cost of use? 

In a sense, we have. The apps on smart phones have a more consistent user interface than web sites. This is due to Apple's and Google's efforts, providing libraries for common UI functions and guidelines for application appearance.

But I don't see a unifying transition for web sites in traditional (desktop) browsers. Each company wants its own look and feel, its own brand presence. It doesn't care that web sites take a long time to load, and it probably doesn't care that web sites require a lot of expensive maintenance. Microsoft was able to sell Windows from a position of strength, in a market that had few options. With the web, any company can set up a web site and offer it to the world. There is no convenient choke point, and there is no company strong enough to offer a user interface that could meet the needs of the myriad web sites in existence.

Which means that we are stuck with large web pages, long download times, and different interfaces for different web sites.