Why is this so?
For any particular cabin assume a very (over)simplified scenario of three fare buckets:
$100 - $200 - $300
An Open ticket can be used in the near future or up to almost a year ahead. The airline has no idea when it might be used, so effectively has to price the ticket at $300 for the return trip, in case they have already sold out of the cheaper seats when the Open ticket coupon is tied to a specific flight. If they priced it any less passengers would just switch from booking the $300 bucket (when that was all that remained available) to an Open ticket.
So if you planned to fly the very next day when there is only one seat left in the cabin, airlines are not going to sell it to you for less than $300 when there is still a good chance that someone travelling at very short notice will want that last seat, even at $300.
On the other hand, if a return date is chosen by the passenger the airline may well price the return as one of the cheaper buckets. "Oh! The first fight out on Jan 2, 2018 – not yet sold even one seat on that flight, $200 will do nicely". Hence relative to say an Open ticket from another airline this airline is more likely to get your business.
Even a modest change fee places some pressure on a passenger to choose a 'sensible' return date so gives airlines some indication of when to close one bucket for further bookings and charge more for the next seat. For example, if enough passengers choose, In January, say mid November as their return dates the airline may decline to offer the $100 bucket price for mid November to anyone booking after January, even if up to January not even one $100 seat has been sold. Put another way, even indication of a general intention is valuable to the airlines who can expand and contract the bucket sizes to suit themselves. It does not matter to the airlines that PAX A booked Nov 15, PAX B booked Nov 16 and PAX C booked Nov 17 and they all changed their reservations if it ends up that PAX A flew Nov 16, PAX B flew Nov 17 and PAX C flew Nov 15 – for these three passengers they still have to provide one seat on each day.
In reality, when not an Open ticket there is often not only a change fee but repricing. That is a fall back for the airline. You choose a date where the $200 bucket is available and they get your business (rather than another airline that offers Open returns that are priced towards a "worst case" scenario for them). Change your mind to another flight where the $200 bucket is still available and there is no change in price from 'repricing'. You are happy and all the airline suffers is some administration cost (covered by the change fee), so they are happy – you'll choose them again!
Change your mind to a much busier flight and "That'll be $100". For you probably still cheaper overall than two singles (even if not as cheap as it might have been had you stuck to your original schedule), each booked at short notice, and for the airline no loss of revenue – they might have been 'guaranteed' to sell 'your' seat for $300 to someone else if not to you, but their revenue for it from you totals $300 anyway.
I suspect another factor of significance is the switch over from paper to e-tickets. It is much easier to track use of e-tickets than it is paper ones since the records are already electronic. For example by visiting a website's page for your booking reference. Hence it is likely a far higher proportion of e-tickets sold lead to occupied seats than was the case for paper tickets. Open tickets were more popular with business travellers than for those on holiday. The latter often had planned an itinerary in great detail to fit in as much as possible in their vacation of limited duration and knew exactly when they had to be back at work. Business travellers often had not enough idea of how long their work away was going to take to be certain of suitable timing for their return flight.
In my experience, very many business travellers with an Open ticket that could not catch the first flight back from that airline, once their work was finished, (or found they would not be able to return directly, say more business elsewhere first) would simply choose a different airline (buy a new single) or book a new destination, or take the train, even hire a car and drive themselves instead.
Being for business naturally they would claim on expenses the other airline's ticket, or the one to the new destination, or the train fare or the car hire cost. This though only after they had returned. But their Open ticket may have been billed months before. This often resulted in a failure to submit the unused return coupon for credit. As mentioned in Difference between electronic ticket and paper ticket? A Paper Ticket is a akin to a bearer instrument, meaning, the paper itself represents value. So one airline was keeping the revenue from a return while only providing travel one way. Often enough that Open returns were quite attractive for them. As mentioned, they became much less so once tracking of unused legs was improved.