There were three major factors that lead to Heathkit's success, which are no longer true.
1) The cost of manufacturing an item was significantly higher than the cost of the parts.
2) Items were sufficiently simple (or at least discrete) that they could be made at home.
3) Electronics were expensive!
Consider that at one point you could order a kit for about 60% of the price of the finished item. This could save you the modern-day equivalent of hundreds of dollars, and you could assemble it in a week or so.
Nowadays, a chunk of electronics is worth about a hundred bucks or so. Turning it into a kit would be _more_ expensive, and would take three minutes to snap together, if it could be done at home at all.