Short Answer: Lower barrier of entry & no quality control.
Long Answer: After the US industry crash of 1983, console manufacturers serve as the gatekeepers to their related platforms. This was a key thing with Nintendo when they released the NES in North America, as they wanted to maintain what they considered "high quality" with their licensing system in order to recover the confidence of the public.
While the requirements to publish a game on a console has been lowered due to legal regulations & competition over the years, every game that gets a commercial release (physically or digitally) must be certified by the console maker (Nintendo, Microsoft or Sony) to meet minimum quality requirements. Because the certification process can take weeks or months, game developers & publishers need to nail this the first time OR risk delaying their game's release (as they have to get certified before their game can go to the publisher's factory).
Additionally, console makers generally require software developers & publishers to purchase a Special Development Kit (aka SDK) that has specs similar to the console it was designed for, so games can be designed & optimized to run on that hardware. While console makers may support game engines (like Unity) to make development easier, they still control the add-on to export the finished game to their platform.
On the PC side, there's no gatekeepers for quality control. Before the commercialization (& public access) of the Internet, developers have to get the attention of a publisher & then a major publication (like a gaming magazine) OR pick up good word-of-mouth through demos & shareware. With the Internet, developers can publish their own games online & with digital distribution platforms like Steam, it's even lowered the barrier between developers & consumers. While some platforms (like GoG.com) are more curated, where they select the games they feature on their platform (to help ensure the "good" games are being promoted). However, developers can go completely indie & offer their game(s) directly to the public, but only games that have some buzz around them or the developer would ever gain some traction in this regard. Most "shovelware" games will try to find themselves on a platform that most people will utilize (like Steam).
There's also the fact that game consoles tend to have a limited market lifespan, which is typically around 6 - 10 years. Once a console has reached the end of it's market life, everybody has likely moved onto the next generation for that console maker. On PC's, while the OS can be a limiting factor on what can run on systems, PC games can be run through emulation (with their original code intact), modified (with a compatibility layer in place) or re-written completely to support newer hardware & OS's. These options can allow games to bypass these limiting factors. Using GoG.com as the example game, some of the classic games they sell originated back in the early 1980's. Ultima I, for example, was originally released in 1981. It was later remade & repackaged as a trilogy set (with II & III packed with it) in 1989, which is the version that GoG.com has adapted to run on modern systems (via DOSBox) & sell on their site.
While Nintendo & other console game developers have updated their old games to run on newer consoles (like the NES Online & SNES Online services for the Switch), it's more like they're reinventing the wheel each time they come out with a new console... so it restricts such offerings.
For the NES & Famicom, there were 715 official games released for it worldwide during it's lifespan
For the PS2, there were 4490 games released for it worldwide during it's lifespan
For PC, there were 1,771 games released on Steam in 2014 alone... This has risen to 9,050 games released on Steam in 2018 according to Statista ( https://www.statista.com/statistics/552623/number-games-released-steam/ ). Over the life of Steam (2004 - 2018), there's been a total of 27,279 titles released on the platform SO FAR (current count of games only at time of writing is currently at 35,321 titles, which makes me a little ashamed that my library contains ~4.5% of their entire games catalog)... this is excluding games that were made for older consoles & content that is likely to never see a listing on Steam (like Minecraft & Sea of Thieves).
I know this is a lot to process, but I hope this sheds some light on the subject.