Because we can? It's not that hard. Nor affording speeders and training with only two gathering profs. Seriously.
I didn't say it was hard, I said it didn't make any sense.
Option 1: Level using a production profession - be able to keep whatever that skill crafts up to date (most useful if you go with synthweaving/armormech/cybertech since they affect the most amount of gear slots), have less overall money due to using up most of your materials instead of selling them.
Option 2: Level using all gathering professions - be able to keep EVERY PIECE OF EQUIPMENT up to date due to having loads of cash, and not just what you can craft for yourself, and still have way more cash left over.
I've leveled characters both ways, through actual crafting and through pure gathering. My gatherer was able to keep himself MUCH more updated gear wise (myself and companion) with MUCH less of a hassle overall.
If you like actually crafting while leveling because you find it fun, that's great. I'm not talking about this from a FUN perspective though, I'm talking about it in terms of pure EFFECTIVENESS.