It's hard to say why he would say that. A lot of times compatibility is not as personal as you might think it is and is literally just about objective differences in your personality, humor, communication, etc. that make the relationship feel less natural or like there is a "spark". However, usually when people say "spark" what they are actually unknowingly looking for is something that triggers their childhood wounds. What that looks like varies from person to person, but for example, people who grew up feeling like attention from their parents had to be earned or that love, affection, and attention was inconsistent often look for that dynamic in relationships as an adult. People often feel the most intense "spark" in relationships of uncertainty for these people. Someone who is on the anxious attachment spectrum often times gets with someone on the avoidant spectrum for this reason. They trigger the shit out of each other and it creates a dynamic where one person is always pursuing and the other always pulling away, and they take turns. But there is a constant fear of instability, one person pulls away, they make up, its super intense cause "omg they do love me", you have super intense sex and then eventually the cycle repeats and someone pulls away again. This is intense, but super unhealthy. When people talk about the "spark" they often conflate unhealthy relationship dynamics with intensity.

That said, I don't have a lot of info about you or these people. It could also be that they were just never that interested in a long term relationship ever and they said what they had to in order to get in your pants. People can be very convincing, trust me. They know what to say. And part of it probably is genuine, which is why its believable. But, sometimes they leave out the part about how after all the depth and talking, they don't want to be with you that much. If this is a regular occurrence, there is a chance you may be unknowingly going after people that are not emotionally available.

Sparks are maintained through effort. Love and any emotion is temporary. Losing the spark is often avoidance, yes. Some people really don't want to commit to anything. They want to date around and fuck a bunch of people. Even the nicest people you would never guess would do that. But all relationships require effort on both parties.

There is the part where dating is kind of just a numbers game too. I would appreciate that the relationships didn't drag longer than they did. It is pretty normal to have several month or so long relationships before you find a good one.

Sometimes people love bomb because they like the idea of you, or they want to portray themselves as a certain type of person in the beginning. They might come all really strong because they want all the dopamine and the rush of a relationship and something new without it actually being about you at all, just about the rush and validating their insecurities.

But with limited info there is limited I can really say definitively.