They can be, but you wouldn't necessarily get what you expect. Hybridizers grow them from seed all the time--that's the way we get new varieties. I think the general rule for hybridizers is you get one variety worth keeping and passing on for every thousand seeds grown. I'm sure more than one of those thousands would be nice enough for most of us, but they'd be too similar to things already out there, and not quite the same so they couldn't be given the same name. I remember a year or two on this forum someone posted a picture of the wheelbarrow loads of beautiful cane begonias being carted to the compost---It's really easy to grow them from cuttings, and you could try the plant exchange to get either cuttings or rooted cuttings I think.