Why do most companies have no special policies for improving
productivity on their workplaces, especially in terms of ergonomics?
I'm not sure about the term 'most' here. Some companies invest in productivity improvements and ergonomics, and others not as much. It's always a matter of degree.
The pressure on management for many companies to show annual or quarterly improvements often skews action resulting in more immediate, rather than longer-term payback. Particularly for public companies, the pressure is high to show quarterly results.
At budget time, many managers have to choose a mix of shorter term expenses versus longer term. It's tempting to decide to invest less in faster computers at the expense of more computers, or to purchase less expensive chairs and instead spend it on software that might avoid having to hire additional accountants, for example.
Most of the time, the ROI isn't at all clear cut. For example, it isn't clear how much benefit the company will gain from spending twice as much on chairs. And how much more productivity is gained by faster computers? Unless the current computers are so slow that people cannot get their jobs done, speeding them up might provide little to no benefit.
At my company, if our computer dies, it is replaced with a new computer from inventory. That has happened to me twice over the last 5 years. The newer computers are much faster - but there is zero difference in my work. I get exactly the same amount of work done with the new computers as I did with the old. I can easily conclude that it would make no sense at all to spend any money on upgrading my computers unless needed for some reason other than productivity. The same reasoning applies to most (but not all) of the computers in my division.
And when the budget is tight, it makes sense to focus on the short-term. If you cannot get through the short-term, then the long-term is irrelevant. (A smart person once told me "in the long term we're all gone anyway")
The companies that handle things well invest in both the long term and the short term in amounts relative to the needs of the employees as well as their external stakeholders and the markets. Some do this better than others.