Why do we buy compost?

Every year we buy compost and have the same discussions about what rubbish it is. Or we make our own (or try too!). 
I can see the benefits of home composting and returning that to the soil but why do we sow seeds in "compost" when we also direct sow into the ground? Why don't we scrape up some garden soil and sown our seeds into that? 
I'm starting to think that commercial "compost" should be renamed "con-post". Can anyone convince me otherwise?

I can't convince you otherwise.

I sow most seeds in or prick out into modules, and soil just isn't a very good growing medium in containers. You can have it as part of a mix, but 100% soil is too heavy, compacts too much and holds on to too much moisture. Now I've moved away from growing a lot of things in containers, I buy much less compost. I only really use it as part of a seed sowing mix, as the castings my wormery produces are too fudgy to use on their own and my homemade compost is full of miscellaneous seeds. The only things I sow direct are peas, carrots, parsnips and certain flowers.

Growing in modules gives me more flexibility - I can start things earlier and keep them indoors or in the greenhouse, they are less likely to be damaged by slugs etc., I can have plants at an already good size for when bed space opens up, it is easier to transplant them without damaging roots, you have more control over germination conditions, there is less thinning out...

If I had a good supply of leaf mould I probably wouldn't buy any compost at all, but we don't have any big deciduous trees in our garden and any leaves are left where they drop in the borders.

I’m hoping to stop buying compost. Or definitely reduce the amount I buy massively. If I get my homemade hot composter working correctly. It’s a big “if” granted.

TMTT has nailed exactly why we buy compost.
Veggie questioning why is incredibly thorough provoking. Especially at a time when peat free is being pushed for environmental reasons. Because from my very limited experience of peat free it’s no better than homemade cold composting. Probably worse as it’s full of plastic and glass (in my personal experience).
Has there been some sort of brainwashing going on to make gardeners need, want compost?
Yes there is the screamingly obvious you can’t make enough compost. This has always been my problem.

This topic is Definitely something that I’ll be thinking about a lot.

I couldn't put it better than TMTT. Plus we're conditioned by the media and experts to grow in compost thinking it will create a beneficial, controlled growing environment with just the right nutrients. In the past compost didn't seem such a financial outlay but with some retail compost up to £9 a bag I dont think I'll be the only one experimenting with DIY growing media this year.

Thanks for your replies.
As you may remember I bought a garage load of Coir last year (being sold off in Morries). I thought I'd be able to start seeds in that but its not working for me. Poor germination and those that do germinate are weak. For the first time in living memory I have NO tomato plants.
Currently, I'm mixing the coir 50/50 with a bag of compost which is giving better results but I've lost confidence in the coir. It may end up as top dressing on beds as I don't want to risk sowing seeds, or planting tomatoes in it.

I have always bought compost for seeds and seedlings because it was sterile: no weed seeds and no nasty bugs.

The current peat-free stuff is still often free of weeds, but it comes with free fungus gnats, needs seiving, needs modification to get the moisture retention right, often needs added fertilizer... and the seeds still often either rot or damp off. Even so-called "John Innes" is poor now that it uses peat substitutes.

I can see me using less and less as time goes on and as I get more diligent in my compost and leaf mould making. I won't be hot composting, so there will be weed seeds in there; but for sowning large seeds and for pricking out / potting on I hope to be self sufficient. So one bag of seed compost a year will hopefully do me.

I have started moving away from twig and plastic compost, ie shop purchased.

I have two dustbins that contain spent compost and vermiculite, I feed it from time to time with riddled compost from my main compost bins and a little liquid feed, worked fine last year and so good so far this.

As JJB says, the cost is getting silly now so I shall hold off buying for as long as possible.

I buy 120ltr of compost that's used for toms, cucs and any other seeds I start of in the house and are staying indoors until its time for them to go out.
It's mostly because the compost I make always has slugs and other seedlings popping up. After that I mix store bought/my compost together for potting on.

Like TTT I also grow in modules/newspaper pots I don't have a lot of space and growing that way means less gaps in the beds.

My garden soil is clay, so using that to sow into would probably not result in very happy seedlings! The seeds and plants I plonked directly into clay last year (out of necessity as I hardly had any compost then) did not do well at all. I'm using bulk compost purchases to get my garden started. After that, I agree that buying lots in isn't ideal. However, unless you are composting everything the garden produces (not eating any!) then you are going to have to bring nutrients and organic matter for drainage in from outside in one form or another.