Unless you're willing to back those claims up with some specific cites, I'd dispute that.
Isometrics have their place, particularly in rehab or for mobility-challenged trainees (say: an arthritis sufferer unable to do proper squats). Or, as you note, when other equipment is wanting.
Given the general principle of SAID (specific adaptation to imposed demand), or more concisely, "specificity", there's a distinct different between a static contraction, and moving a joint through a full range of motion against resistance in both concentric and eccentric phases.
The short answer is "because it's what's been shown to work" (where "it" is full ROM training).
"Specificity" refers to the adaptation of muscle to stimulus, and I was answering specifically why isometric and full-ROM training are different and will have different results: They will differ in response because they differ in stimulus.
If you want to read detailed technical literature, I'd suggest you do your own Google Scholar search on hypertrophic response from isometric vs. full-ROM training modalities. I'm not particularly interested in the results.
What I know is that there have been few serious bodybuilders (the guys who are most interested in training efficiency in developing more muscle mass) over the past 75-100 years who've advocated isometrics. Your standout oddballs are Herschell Walker (and he pimps calisthenics, not isometrics) and Charles Atlas (a modified "dynamic tension" model, not strictly isometric). There are occasional iso advocates, but they're well off the mainstream.
So: if you're currently untrained, and you don't have any limitations on full-ROM training modalities, your best bet is to do a freeweight, compound-movement, whole-body training program, about 3x weekly. This will do the most to add muscle to your body with the least amount of training and effort. Standard modifications would be to substitute a two-part split and 4x schedule, possibly 5x, if you have the time for it. Training volume, recovery needs, and training frequency won't let you go much beyond this. And retain an advantace over the first option.
If you're interested in disputing this, I'd encourage you to research the claim yourself (it's a pretty conservative and widely accepted one), find its critics, and judge their arguments and data on their merits. I'm not particularly interested in the question at this point.
Isometrics have their place, particularly in rehab or for mobility-challenged trainees (say: an arthritis sufferer unable to do proper squats). Or, as you note, when other equipment is wanting.
Given the general principle of SAID (specific adaptation to imposed demand), or more concisely, "specificity", there's a distinct different between a static contraction, and moving a joint through a full range of motion against resistance in both concentric and eccentric phases.