I believe that giving higher discount for buying all the games of a serie vs only 1 game while conceptually correct might also be counterproductive for GOG.
I wanted to buy Icewind Dale remastered but if I select only that game the prize is higher. I already have both BG and BG2 remastered on steam and I won't buy them again.
I decided not to buy the game and wait for a higher discount on the single game or a good discount on Steam.

Buy them as gifts, redeem Icewind Dale, trade the Baldur's Gates.

I'm OK with it, since they don't force you to buy the whole bundle again (assuming you owned it here). It royaly pisses me off when steam offers bundles with high discounts but force you to rebuy stuff you already own (without even giving you the rebought stuff as gifts)

As a rule, I tend to have games all over the place. I buy only when I'm interested in, but if it's bundled in a way that's cheaper than getting just what I'm after, then I don't mind spending a little more for the discount.

Are these remastered versions really worth the money ?

You can use them to build classic versions with voice acting, so mostly yes...

I think it's also because that way people buy the whole bundle and not only one or two handful games as it's cheaper that way (and go 'well, since the two games almost cost as much as all five, I'll fork over the other 2-5 dollars for the whole bundle"), and so overall they make more revenue.

So if you imagine people buying let's say two games for 1.99 versus a whole bundle for 9.99, even if you have less people buying the bundles than you would've for the individual games, you usually profit from it. Especially in case like this when it's one of the best discounts next to Insomnia and Weekend sales.

It's basic marketing. Get a larger discount for buying more - kind of like buying in bulk.

GOG doesn't know if you own some of the games on Steam, nor would they care even if they did. Why would they offer a higher discount to someone that bought games from a competitor? As with any purchase, it's up to the buyer to decide if the discount they do get is enough to pull the trigger. If not, then they wait for something better, which is what you've decided to do. Sure, it may mean GOG 'loses' a few sales, but it's highly unlikely to be a large percentage of interested customers.

Ah, the obligatory "It isn't discount-y enough" thread.

If one insists on getting the max discount then I like Randalator's idea: buy all and gift or trade the leftovers.

Getting it gited by someone would also be an alternative...

Ohh, good idea. There are sure to be plenty of those threads popping up soon.