There are a number of forces at play in this, but one of the important phenomena at work is the concept of candidate differentiation. During a campaign to be elected, a candidate is almost universally advised to draw distinctions between themself and their opponent(s) so as to help voters perceive a meaningful choice between the candidates. That distinction is then framed in ways that appeal to their target voters.
This research from the Pew Center highlights the sharp differences in polling on social issues (in this case institutional/structural racism) vs. the relatively muddled waters of economic issues (the page before it).
So an answer to your question is: "Because that's how candidates are being packaged and sold to voters."
The perception you ask about also arises as a function of what media you consume (and by extension what media various social media algorithms feed you), who you discuss politics with, and how you, personally, frame a given issue.
Take homelessness as an issue, for example:
This can be approached as a social issue, by assigning moral weight to someone having descended into homelessness in the first place; an economic issue, by discussing it in terms of economic inequality (which is itself possibly a social issue); a fiscal issue, by discussing the relative costs of providing housing vs. providing various forms of medical care or policing to react to the population's presence; and so on.
The divide between 'social' and 'economic' has never been a very distinct one - is immigration about the identity of the United States as a nation? Or is it about jobs and resources being distributed among the population? Is marriage about equality or tax benefits? Was the first Gulf War about the independence of small nations, or about US access to oil from the Middle East?
The answer to these questions is usually, "Yes."
Insofar as things are being framed more in terms of ideology than they are in terms of cost-benefit, the relationship there is bidirectional with how voters are thinking and communicating about these issues.