Why do so many people live in Ontario and the rest of Canada is practically bare?

You are right of course

This is Canada

At present, about 37 million people live here. 

In that orange blotch, Ontario holds about 14 million of them. However, of those 14 million, over 6 million live within the boundaries of this map

And if you go a little further, the area defined here as the “Golden Horseshoe” contains over 9 million people - about one in every four Canadians

This, of course, is a tiny tiny fraction of Ontario. Here’s the same region overlaid on a map of Ontario

So, why here? The reasons are largely historical

• That blue line over on the right is the St. Lawrence River, which leads to Montreal and the Atlantic Ocean. That provided a way for people to get to that region and to get goods out of that region. That lake at the right, Lake Ontario, also made it easy to get around the region by ship, allowing everyone in that region to trade with each other.
• This region has a lot of rivers which, in the early days, were used as trade routes. for example, the Humber River empties into Lake Ontario near central Toronto. The First Nations and early settlers used the Humber as a shortcut to Georgian Bay (the bit of the lake that touches the green portion at the top. As the source of the Humber is very close to the source of another river that goes to Georgian Bay. There are lots of rivers like that in the area that allowed people to move goods easily.
• Most of the Golden Horseshoe was productive farmland or forest, which is why people settled there first. There was also productive farmland to the west, but the rivers there flowed into Lake Huron or Lake Eire making it harder to move goods to the Atlantic. That bit to the East does have some farmland, but it’s also got a lot of lakes that usually got in the way - the rivers there run a bit inconveniently too - you have to go west to Kingston (where the lake meets the river roughly) before you can head east again. Most of the north isn’t farmable - it’s part of the Canadian Shield and is very rocky.
• That bit of river next to the bottom of the red area is the Niagara River. In the 19th century, it was an important source of power for the growing region. By the early 20th century, Toronto was fully electrified and started growing much faster.
• These advantages meant that when transportation started getting more modern, a lot of money was spent here even though it’s a bit “out of the way”. One of the first railways in the province joined Toronto to Georgian Bay at Collingwood, which became a major shipbuilding centre. A canal was built linking Lake Ontario to Lake Erie. The area became more industrial as it was easy to ship raw materials over the lakes. Toronto, the main port due to its natural harbour and railway connections, became a major shipping centre for grain, sugar and meat in both directions. Industry started to take off in Toronto, Hamilton and Oshawa thanks to their lake access and railway networks, which were soon extended west to Buffalo, Port Huron, and Detroit.