It's because the body is extremely good at adapting to whatever you do to it. Even if you were riding for a hundred hours a week at maximum aerobic output, your body would slowly adapt and you would stop making gains (getting faster, losing weight).
Over a long period of exercise, you have to constantly vary what you are doing to your body so it won't adapt and plateau. Keep throwing different challenges at your body, and it will keep adapting in an attempt to deal with these new challenges.
Try sprinting home from work at max output a couple of days a week, then intentionally go for an extremely long ride on the weekend at a sustainable pace. Don't do the same thing day-in, day-out
Also, eat less calories if you want to lose weight.
this is the reason elite athletes keep getting better, they keep up intensity, continuing to push the limits. Bike committing gets easier, and, like you said, your body adjusts to the level of output you need to get to work.
Sure, deciding to bike commute isn't a guarantee of sustainable weight loss. But no single activity is. Combine its caloric deficit with a sensible health plan and you'll reap its benefits.
20km/h : human radar scanning hidden corners. no more looking at girls or you'll hug trees.
30km/h+: better have swift brakes, any surprise feels like a threat.
First, as time goes on, you care a lot more about safety. Newbies may write about how they can meditate or listen to music while riding, but experienced riders know they must always stay alert. You shouldn't be meditating, you should be focused on potential dangers.
Second, the weight loss? Not as big a deal as time goes on. Don't ask me the medical reason why, maybe it's the same phenomenon as when people who haven't worked out for years start a program and quickly lose 10 pounds of water weight but then find the rest is harder to lose. Commuting usually doesn't involve the intense aerobic activity associated with road racing.
The 300 dollar bike you are satisfied with? I didn't stay satisfied after a few months. After trying many options (too many) I took Sheldon Brown's advice and ride a touring bike, with disc brakes, fenders, rack, and panniers.