Viruses are also very specialized molecular machinery, and eukaryotic cells (and by extension, organisms) are quite complex. Think of molecular evolution between viruses v. prokaryotes / eukaryotes like a constant arms race: viruses have the "key" to (1) enter a cell and (2) hijack cellular machinery, and prokaryotes / eukaryotes are constantly upgrading their locks (mostly as a factor of diversity x natural selection --- the cells without great locks tend to be terminated rather quickly).

Also consider that it's not in a virus' "best interest" (from a sustainability / propagative standpoint) to actually kill or even incapacitate 100% of their hosts, which contradicts their very mode of replication (often completely hijacking transcription and translation mechanisms in a cell, leading to lysis). They rely on these hosts to survive --- a 100% termination rate of the host means an evolutionary dead-end for a virus.

Arguably, the most "successful" viruses are ones that have been incorporated into host DNA - it's estimated that up to 8% of our genome consists of endogenous retrovirus DNA [1]. After all, viruses are simply small containers of self-replicative material (DNA / RNA, a few proteins) surrounded by a protein coat: if they can hijack their own environmental shield (like, say, a full human nucleus / cell) and still propagate their DNA they've arguably "won."

So --- yeah, harmful virulence is (1) tough to achieve and (2) not actually all that evolutionarily advantageous.

[1] https://en.wikipedia.org/wiki/Endogenous_retrovirus