I think the word “rare” is a big contributor in discussion like this. If the cards were just “secret” but not “secret rare” I doubt anyone would think twice about there being so many more of them included in sets today as rarity itself wouldn’t be implied. When the sets are printed in such large numbers to the point where it feels like everyone has these cards, the implied rarity gained from the presence of the word “rare” somewhat loses its meaning when looking at the cards out of context of the set specifically.
Splitting the two words apart, “secret” tells us that it’s not included as part of the main numbered set and “rare” tells us that it’s not easy to pull from packs - which it quite rightly isn’t. Looking at it from this perspective shows that it still conveys the same meaning it had all the way back when Dark Raichu appeared in the Team Rocket set.
What has definitely changed however is the concept of it being anything special. Pulling the Dark Raichu from Team Rocket meant you’d found the secret card, but now there are so many of these secret cards that the satisfaction of pulling one is greatly saturated and isn’t much of an accomplishment any more. On top of that, the over-use of the repetitive and often lacklustre rainbow holofoil pattern has driven the cards which aren’t secret rares in their respective sets to command higher monetary values.