The dominant direction of web development for the past decade has been app-ification: everything becomes a product, products require accounts, accounts require email addresses, email addresses become marketing infrastructure. This direction is so consistent that the sites that refuse it have become conspicuous by their resistance.
The sites in this category tend to share a few characteristics. They load instantly. They work without JavaScript enabled, or degrade gracefully if JavaScript is disabled. They have no login wall. They don't ask for your email. They often have one-page designs that load everything at once. They sometimes look like websites from 2004, and they're not embarrassed about that.
A small taxonomy
There are a few distinct types. The utility category includes tools that do one specific thing — convert a file format, generate a color palette, calculate something obscure — and have no interest in becoming anything more. These are often made by a single person who needed the tool themselves.
The art category is weirder. Sites that are built around a specific aesthetic experience: a generative drawing tool with no save function, an interactive poem, a sound toy. These are often built by people with design or art backgrounds who find the constraints of the pure web — no installation, no persistence, immediate access — interesting rather than limiting.
The third category is reference sites: exhaustive catalogs of something specific, maintained with the same energy as a printed encyclopedia. These are the most fragile because they depend on a single maintainer, but also the most valuable when they're good.
Why this is happening now
Part of what's driving this is the maturation of the indie web community, which has been building tooling and community infrastructure that makes it easier to build and maintain simple sites. Part of it is a straightforward reaction to the increasingly hostile experience of using most web applications. And part of it is probably just that some people have always been building this way and it's become more visible as the contrast with mainstream web development has become more stark.