Of course it matters. But that doesn't mean (very moderately) less dense softwoods are not fit for purpose. In home construction, you're either loading softwoods in compression along the grain or you're using thick material (or engineered ones, which are Lignins Georg and should not be counted) as beams.
Look at https://www.wood-database.com if you want numbers. Balsa is around 9 lb/ft^3 dense, with a crush strength of 1,690 lbf/in^2 and a modulus of rupture around 2840 lbf/in^2. White spruce--which is a pretty representative member of "SPF" in the USA--is about 27 lb/ft^3 even in its modern, "oh so weak" form, with a crush strength of 4,730 lbf/in^2 and a modulus of rupture around 8,640 lbf/in^2. That's, uh...a lot. Is it Good Old Hardwood? No, but who cares? The numbers we're talking about are so strong that if you need more, you need a big span or something, you throw a 2x12 at it or you go to engineered materials (which are basically wood alchemy, they're magic).
From a numbers perspective, the modern "oh they take so much off" 2x4 is overkill for things like external wall framing. We use them because of safety margins and because it allows us to box in more insulation.
Where it can matter is in terms of rot resistance. Sort of. Heartwoods do have accumulated waste products (they're basically dead while the tree is standing, if you ever see a tree desleeve it's because there's too much heartwood for the sapwood and it just peels off) that bugs don't like. And you can tell why; if you've ever bought a decent-sized board, you can taste the difference if you lick your finger, touch the wood, and lick your finger again. Heartwoods are bitter. There is value in that, and old growth does have more heartwood. But we have modern building practices that maintain a better moisture envelope not "because the wood is worse", but because it makes for a nicer house to live in. It also happens to compensate pretty well for that.
The people who fulminate about "the wood isn't as strong" are old and old at heart. It is not just "strong", but it is strong enough, and the fact that we can produce so much softwood for so cheap at such truly incredible strengths is worth celebrating.
Myself and friends are using 200-400 year old growth jarrah in our building today - it was sourced from treefall in the 1980's, slabbed and stacked in sheds on farms for retirement.
Jarrah was used to pave high traffic thoroughfares in London at the turn of last century.
Once our state stopped the wholesale clear felling of jarrah by europeans, we turned to harvesting regrowth:
Jarrah is a sustainable, exclusively sourced from regrowth forests in Western Australia. Strict regulations are in place to ensure jarrah sustainability is managed in a way that balances ecological, economical socio-cultural factors.
The guidelines and policies outlined in the The Forest Management Plan and the WA Regional Forest Agreement are regularly reviewed to ensure world-class standards are upheld. All native forests that are harvested in Western Australia are regenerated or replanted every year. Jarrah can be recycled and repurposed for other uses.
Even though Jarrah is very sustainable timber, the West Australian Government has made the decision to ban logging any native timbers by the end of 2023.
Exposed beams with detailing look great, not to mention the milage you can get using hybrid techniques such as steel I-beams masked with long thin jarrah inserts.
Natural edged shelves, island tops, tables.
South-West W.Australia has a lot of old craft skills kicking about - I was in the midst of a crowd doing glass blowing, cabinet making, solid and ply shell drums when I was writing pre-Google Earth not exactly KeyHole geo spatial processing and display software and we'd often going out to retrieve all manner of wood and rock that was toppled or exposed by storms.
It's not just jarrah stashed away, there's also wandoo, granite slabs, numerous burls extracted .. and a lot put back in terms of replanting, cool burns, 'artifical' hollows, culling ferals, etc.
NO DOUBT, I am lucky to live in central coast and my house is made of it. I think the post a couple up was talking about moisture and rot resistance tho.
Edit: just says old growth ... i thought i saw redwood
