Because lenders believe that people who borrow more than the amount they owe on their current mortgage are more likely to default. If you're likely to run up balances on your credit cards again, you may end up in BIG trouble. You'll have even more bills to pay, less home equity, and you might even face the threat of foreclosure. Never take out a loan based only on the monthly payments. And while a fabulous family vacation sounds awfully appealing, the great memories could quickly fade if times get tough and you're mired in debt.
Lenders are happy to do it.
You can't count on the lender to do that. I doubt it. Take out a 30-year mortgage, and pay it off like it's a 15 year. If you do ever run into rocky times, you can slack off and just make the regular payments for a while, without worrying about getting behind on your mortgage. Invest in Yourself won't tell you how to make a lot of money, but it will explain how you can live better on less than you think while saving money.