Their explanation is that it messes with and over complicates their LOD(level of detail) system. What that means is when you zoom out, objects/scenery drop in detail both in geometry and texture so you're not dealing with a lot of excessive rendering at a zoomed out level. If you had an object say a rock, scaled much bigger, then you'd have to take into account its LOD at a larger scale when zooming out. You don't want this massive rock you scaled larger to shrink back down to its original shape upon zooming out due to LOD issues.
With Sims 4, they're just dealing with one house and a static neighborhood. With Planet Coaster, you have an entire park to render that expands constantly. Having to program multiple LOD's with the scale of X object would just add even more work and resources.
Also, Sims 4 has a 20 some limit on Sims and each house/district has to be loaded separately. Planet Coaster so far has a 3000 limit on their peeps. You're comparing apples to oranges IMO by comparing PC to Sims 4.
IN A NUT SHELL: Frontier didn't develop a system for scalable objects. It's totally doable but it would require a ton of work to (re)configure LOD's and sizes of objects/their textures & geometry.