You Hate the Idea of Moving
Change is hard - most people are change averse just as a by-product of being human. Many people relocate out of necessity and quickly find out that moving is stressful, costly, and something you want to do as few times as possible over a lifetime. Many homeowners have said over and over again that they ultimately decided to renovate because moving simply was not a realistic option. Many were already too close to family and/or too close to work, so it seemed impractical to give up those advantages for a larger home which may also need some renovating of its own soon. The end result was to stay where they are, renovate, and not have to worry about moving for a very long time.
Lifestyle is Important
What was once touted as "keeping up with the Joneses" is now recognized as a legitimate exercise in maintaining value and increasing happiness. In many cases, home improvements, and the subsequent lifestyle increase, tend to add real monetary value to your property, not just the intangible positive feeling of enjoying your newly renovated space. Lifestyle is important - homeowners recognize they've worked hard for their money - so it only makes sense they should spend some of it where they spend the bulk of their time... on their property.
"Do It Yourself" Projects Are Time Consuming, Stressful, and (Typically) Done Poorly.
You may have heard the expression "you get what you pay for" more than a few times in the past. This is particularly true in the home improvement & construction industry.
You Absolutely Love Your Location
Some homeowners will keep making more improvements to their existing home because their location is simply too good to ever leave. Renovations in this context often become more cosmetic and for modernization (rather than typical home improvements - for example, where kitchens are enlarged or basements developed for more practical living space.)
This remains the top reason customers eventually decide to take the plunge and renovate: it simply makes sense to do so. Perhaps your family has outgrown your outdated bungalow, and you need the basement space for the kids as they get older. Or maybe you love to cook and entertain in your kitchen area many times each week, but you've found that the kitchen layout is simply too closed off for these gatherings. Another possibility is a home addition over the garage for an aging relative or just as some additional rental income. Whatever the reason, homeowners have a practical concern which requires a practical solution from a professional renovator.
Of course, many times, people eventually decide to renovate as a result of all 5 factors above, or some other combination of them. No two homeowners are ever exactly alike.