>>103740787They were there sort of as stock character imaginary waifu archetypes originally, but clearly with the potential for depth included--it's just that Finn was a pubescent boy with a huge crush on one and a sort-of-crush on the other (that blossomed into a cool spooky warrior bromance), and we viewed them from his perspective.
Their increasing depth was, by an large, a good thing for the series, and made sense: Finn got older and wiser, and started seeing them in a less idealized light. Their flaws and their strength became more apparent to him, especially after he moved on from his crush on Bubblegum.
The problem only came about when Finn's toxic relationship troubles became a focal point of the show for a while, then the writers stopped writing heroic and ADVENTUROUS stuff for Finn to do. His "bit" in the show became being a sadsack or a creep who needed to mature and learn things, but in developing these aspects of him, they largely abandoned the aspects people liked about him: unflinching (and sometimes overzealous or overly-violent) heroism, excitability about little things, an ability to put aside angst and uncertainty to Get Things Done. They made him arguably more realistic for a young boy (complete with suddenly being totally out-of-his depth fighting supernatural monsters), but forgot that nobody liked Finn or the show because of its realism. They originally tuned in to watch a boy and his magic dog brother explore a bizarre and terrifying world with a sense of wonder and confusion.
Around the same time, to make up for the deficit of fun and adventure around Finn, the show started doing more and more episodes focused around characters the writers still liked and considered "realistic" action protagonists for their setting, who were mostly superpowered uberbeings like PB and Marceline. These characters arguably got overdeveloped as a result, and stole all the focus from Finn and Jake. This persisted until the finale.
