When a spec home builder or tract home builder builds a new house they are optimizing the house design and specifications for the average homebuyer. The result is a lot of homes that look very cookie-cutter and all have very similar specs. This works really well if every choice you want to make is at the median but if you have specific tastes that are at the tail ends of the distribution then you are going to be out of luck. Many people find themselves in this situation, where they want to customize certain aspects of their home design beyond what the median consumer would want which is what makes custom home design and construction a great choice.
Regardless of where you stand on green energy and sustainability, it's easy to see that we're collectively underinvesting in power generation which will lead to significantly higher heating and cooling costs in the coming decades. Most spec homebuilders are insulating houses to the bare minimum and installing HVAC systems that are not highly efficient. There's always a tradeoff on cost but it pays to increase air-sealing, window glazing quality and certain types of insulation.
Grid stability can be a challenge anywhere so many homeowners prioritize the ability to independently retain power and keep their family warm. There's no better time to plan for adding backup generators, batteries or solar than the design stage. It can be both cheaper and result in a system that operates better when you need it.
Reducing home maintenance is one of the primary reasons people buy a new home but not all newly constructed homes are equal when it comes to preventative maintenance activities. Non-custom home builders have very limited incentives to reduce maintenance costs beyond when they sell the house to you. Instead, with a custom home you can optimize the design and specifications, both inside and out, to build a house that won't need as much maintenance.
People who build custom homes often have a focus on the future. The home is often designed to be a family heirloom that can be passed down and enjoyed by all. A non-custom builder has a different focus when building a house, they build it to meet minimum code and pass local building inspections. Beyond that, their focus is only to make the visible aspects of the build look good so they can sell it quickly. The problem is that the invisible aspects (foundation, drainage, framing, ducting, plumbing, electrical, etc) are all extremely important and extremely expensive if not done properly. These vital systems can fail down the road and leave you or our descendants with difficult choices to make. When you go with a custom home design, you can specify better core systems and oversee every aspect of construction to ensure it's done properly.
Let's be honest, a spec home builder has no incentive to design your house to be future proof. If you want to ensure that your house is going to remain accessible as you age in place, or will be able to be cost-effectively retrofitted with new technology then you're going to want to consider a custom build so you can design your home to be exactly what you need it to be.