Banks are business, that like many business sell products and services and make profits.
In life to have the most amount of options and savings, sometimes you need more then one company. This is true for mortgages as not every company will have a mortgage on sale at the same time. Just like not every grocery store will have chicken breast on sale at the same time. The more grocery stores to choose from, the more options you have and therefore more savings. Staying loyal to one bank can be comparable to staying loyal one grocery store, clothing brand, hotel chain or car company. You would start to feel limitations if you could only ever shop at one clothing store or drive one brand of car for the next 25 years.
I may have access to your very own bank and can explore unadvertised rates and exclusive offers with them so you can stay there if you prefer but enjoy a better rate.
With your credit card, personal loans and day to day banking, it’s great to be with one bank. This would be for convenience and having everything on one smartphone app. However, it may cost you more if you don’t expand past just one bank for your mortgage shopping. A mortgage holder usually visits their mortgage a mortgage every 5 years at renewal. It doesn’t need to be like your credit card where having one company forever is best. For example, when collecting travel points.
You can be loyal to your mortgage broker, just like you may to your stock broker, or company like Booking.com. That broker can be your one stop shop, but within that one stop shop you can use all different companies. Therefore, whoever has the best rate and options for your needs at the time.
A stock broker will look to put you in the most profitable stocks. Usually this means more then one company in your portfolio. For convenience however, you have the one person to call. That is the stock broker and they can manage all the options to save you more. If only allowed to invest your money in one company your whole life, you may not see the gains possible from having access to different companies.