They also overpay many of those players in their first 6 years as well. And right now the determination of worth appears to be an arbitrary process at best, and probably not one agreed upon by both sides. You present your argument as if it is a clearly recognized fact along the lines of "water is wet." Simple question - which is greater - money underpaid to pre-arb players or overpaid to free agents?
Also the way the arbitration process is heading, the values are rapidly approaching the same numbers in free agency. Only 9 players have a higher salary in 2018 than Josh Donaldson received in his last year of eligibility.
Not to mention, owners this year have most definitely been making offers. Wade Davisset a record for AAV on a closer. Drew Smiley and Michael Pineda each received two year deals when they were both out the first seasons of them (effectively making them high risk one year deals). Juan Nicasio, Pat Neshek, Tommy Hunter, Brandon Morrow and Addison Reed all received very high AAVs for non-closing releivers, but for fewer years. The AAV for each of these non-closers is among MLB best for non-closing relievers. While relief pitchers appear to have embraced this idea, the position players have not, without regards for how high the salary is.