Because most weld jobs turn out shitty. Mount kits give the motor much better engine stability and axle geometry compared with weld ins.

Also the way everyone does the weld ins is they mig weld the bracket to sheetmetal. This is not a very strong weld. Tig welded or spot welded (like how they are from the factory) would be much better.

Auto to manual conversion is the big thing you need to worry about, it requires alot of parts that aadd up in price and someone knowledgeable enough to do it.

Good Luck.