Yet, the reality is that even in these companies there's a notion of traction, and it's absolutely critical. It's not measured in terms of users, but instead manifest in scientific milestones. Every time you demonstrate the scientific validity of a piece of your tech, you are de-risked in the eyes of investors. That mirrors traction, even if it's less direct.
Another way that biotech companies can achieve notions of traction comparable to tech companies is through intermediate business models, such as operating as a contract research organization or reference laboratory.
Despite those parallels, I'd say that biotech traction is still difficult to achieve because most science lacks determinism in the rate of progress.
Biomed startups have mostly technology risks, the problems are well-known and there is a very defined (and heavily regulated) goto market process. As the comment above mentions, the thing that gets investors excited in biomed startups is mitigation of the technology risk.