Among other reasons, because the decoder-only version of the original transformer architecture has proven weirdly resistant to these kinds of hacks and clever optimizations.
Ideas like sparse attention, tree attention, residual attention, etc, all sound good on paper, but when researchers try to reproduce them they either find no results or results that don't scale. Even AliBi is turning out to be less powerful than scaled-down positional embeddings. It's almost a bitter lesson on its own: you can't beat the original transformer.
Optimizations that do stick around tend to be the ones that preserve the original algorithm but help with caching or memory accesses.
Because there are a thousand ideas a minute in this field that meet the "it's worth trying" bar but don't actually pan out to make any difference. It's the equivalent of a blogpost that says "if someone else turned my idea into a business, it would be a billion dollar business. But I won't bother."
With say system architecture, you can muse on stuff like "well if Kubernetes made this decision, it would definitely be more secure" or "it would scale up quicker" without empirical evidence and other people could argue "yes I agree because" or "no I don't because"... etc.
With large ML models, there probably is no intuition like this. We just don't know "if I do the common sense thing X, it surely will produce better results for a given benchmark" ... well we have no idea until it is tried out.
