How a web browser is different is that simply by stumbling into some web site, your browser is suddenly loading code from ten other domains. The current version of all that code, not the one the site tested with.

No signatures are checked; for none of those sites is there a validation whether it came from a trusted publisher and a prompt to install it.

With regular software, you explicitly download and install and in newer operating systems, there are certificates and signatures to give some modicum of protection. If you install something unsigned, you give your explicit consent.