I've noticed a few other titles that follow the same pattern, but I can't recall which ones I've stumbled on. As a point in case, consider Raptor on Steam vs Raptor on GoG.

I wonder how much of this pricing policy has to do with customer loyalty and how much of it is blatant exploitation. I should think different people prefer the Steam version for various reasons, including the fact that they don't shop on GoG, have possibly never heard of GoG or are just plain stupid in thinking that the Steam copy is somehow superior or more exclusive. Or are just hipsters who think that owning a copy on Steam will somehow elevate their nerd cred.

There may also be several practical motives for this acute difference in pricing, among them when an agreement was signed with either platform and what kinds of distribution rights/responsibilities either platform is tied to internally. If it were just Raptor, I'd be willing to accept that it's just a crack in bureaucracy and 3D Realms (or whoever owns the rights to Raptor these days) have negotiated a special deal whereupon they simply increased the price on Steam (to be fair, I haven't checked any other platforms the game is likely to be available on). However, since more and more of these exceptions have cropped up as of recent, I can't help but think that it's either a new fad or Steam is seriously inflating their prices and consequently taking advantage of their position on the market. Not that these old games would appeal to the majority of Gen Z, but still - the price difference is more than seven fold. (a stupid oversight)

For the sake of discussion, I'll leave the last paragraph intact:

Personally I don't have an opinion on whether a game that was made 21 years ago should/could/might be priced the same as new titles. On the one hand a game is a game and if it has stood the test of time, its inherent entertainment value hasn't necessarily amortized. A player is paying for more than just a copy of the software - they're paying for the time they spend using the software and the value they get out of it (or don't). On the other hand, come on - it's a simple top down shooter that may have been pristine when it was released, but is plainly dated today. The latter I know to be the case, because I've actually beaten the game and experienced its fantastically one-sided upgrade system first-hand. This happened two years ago, so it's not even that distant a memory. That being said, I couldn't weight in on other similar examples.

What's your opinion on this?

The publisher sets the price of their games, not Steam.

[edit] for me, the links you posted are for a single game on GOG and a pack of 32 games on steam.
The issue seems to be that you dont havr the choice to buy Raptor individually on steam; only as part of a bundle.

Haha - well that was embarrassing :).

While I can officially declare myself blind now, the bigger question remains: would the price of a game be justified to remain the same after a decade or two under any circumstances? After all, while the technology has moved on, this doesn't necessarily mean that the game has grown old because of it, lost its entertainment value or that the effort that went into it is somehow smaller or inferior.

If I put a game on Steam I can't set the price?

Yes you can. Valve might give some pricing advice, but they leave the final decision to you.