I used to freelance using a lot of those sites, I'd often overbid, and here are the reasons why:
1 - Bad project description
If I don't see a precise project description and scope, that means I'd have to get that information from you, which is added time that I'm going to charge you for. For example, the client asks to create a simple one-pager site and doesn't mention hosting services, domain, or support. That means I'll have to ask and guide the client through all those steps.
2 - Most clients have no clue of what a project actually cost
For example, a client asks for a clothing e-commerce. It says it will need a product page, cart page, home page, and checkout page.
But I know there are many extra hidden pages, e.g., admin page to add products, payment control page to issue refunds, force approve payments, include discounts, taxonomy, payment gateway integration, etc.
3 - Clients haggle, and budgets are usually ignored
A client assumes that if he says he only has 300$ for said job, it will be done for that price. I tend to bid for what I'd do for that project and break down the price if the client asks; sometimes, they aren't aware of the actual costs. Sometimes they forget to fill in the proper budget. And sometimes they can get a few more hundred dollars in if I show that I've got a good service to deliver. As a freelancer, I have no way of differentiating between those people, so I bid what it would cost for me to execute the project, regardless.
4 - The project seemed like the back and forth kind
Any project that involves client input on style is a nightmare for me; some clients are unrealistic with their expectations, some other clients want to fine-tune every corner of the page so that they can say that it was their idea. Hey, no problem, but you gotta pay for it. Hence, overbidding.
5 - The project was boring/uninteresting, but I'd still do it if well paid
As a technologist, I'm not a big fan of doing the same project over and over again as it doesn't really add much to my skills, so if it's a project I've done multiple times, I tend to charge more.
One, because I'd be doing it for the money. Two, I did it before, and I already know the pitfalls, so I'll deliver a better product faster.
6 - It's a clone of x / client clearly didn't think it through
It's not unusual to see clients asking for 'I want a clone of Uber, but for classic cars.'
I then bid a huge amount; one is to discourage clones, and two because people have no clue of the amount of technology and work involved in any of those projects.
I had to explain once for a guy that wanted a uber for taxis that it wasn't just one app; it was two apps (driver, client) and a full fledge dashboard for the crew operating the business.
Sometimes the client didn't think it through enough. As a developer, it's my job to deliver code that will solve your problem, not just code what you asked me for.
So, here's what you do, ask the overbidders why they overbid. Freelancers are very frisky about getting locked into a bad deal, and if you can show them some common sense, that you're aware of what you're asking for and that they are not going to be enslaved by a poorly written project description, you will see the prices drop.