I believe it's because Walt and Jessie are the cause of the situation. Walt showed obvious displeasure in having Gale to work with rather than Jessie. He was bored with the guy and didn't vibe with him. I'd even go so far as to say he missed Jesse. The entire show displays a constant male role model relationship between Walt and Jessie. For walt, who had, for the time being, beaten cancer and had no outrageous bills eating up his income, it was a small price to pay. On top of that, Jessie was a person Walt controlled in any way he could. Walt would want to keep him close. Also, Gale was an experienced, schooled, qualified chemist who was deserving of his own income. Gus had actually paid to train him to be his main chemist. Jessie really doesnt deserve his own salary to be even what it is. He's basically just a laborer.