Here are two possible causes.

First, “design” became a trendy phrase thanks to IDEO et al. Everyone wanted to “design” things, and call themselves a designer. Putting post-its on a wall and speaking the jargon now qualifies one more than creating a sound layout.

Second, making web pages has somehow become synonymous with design. Someone who does user research is a designer. Someone who runs a thousand statistically unsound A/B tests to make a button green instead of red is a designer. Nevermind skill/taste/craft pushing pixels with a mouse. We could also blame an obsession with data, metrics, and pseudo-scientific processes — which make individual ability, as well as aesthetics, apparently irrelevant.

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