There's a few factors:
1) no matter how much production you throw at a task, any queue can only do a maximum of 1 thing per turn. This makes the phenomenal output bonuses from buildings completely meaningless by about T3, and adding any new buildings past the 1-item-per-turn limit is pointless.
2) Colonizing another planet is, by far and away, the fastest way to grow your total population. Every planet grows by +0.1 pop per turn. Buildings to increase this number increase it by a very small % of 0.1 per turn. So if you build a hospital, your colony grows by 0.11 pop per turn; if you build a colony (for a similar expense), then you'll get 0.2 per turn.
3) Large Empire Penalty is hopeless at containing expansion. The approval penalty is far too low to actually hurt expanding empires, and simultaneously becomes too expensive to counter at a ridiculously low level - around 20 colonies. So there's effectively no penalty past 20 colonies, and the sooner you get there the better.
4) Trade resources (the ones that show up on planets) are overpowered, and stack. Same with artifacts. By getting 4-5 of each, you can give every planet the equivalent of multiple buildings, for free, automatically.
5) Buildings cost maintenance, colonies capitals don't. Colony capitals are way more powerful then any building, however. So you can either build buildings, and so be forced to generate and spend money on maintenance, or you can just keep adding colony capitals to gain huge amounts of free production which can all be directed at manufacturing, or research, or economy as needed.
These all combine to make 'tall' (highly developed) empires massively weaker than 'wide' (lots of poorly-developed planets) empires. Basically, you may as well check the number of planets everyone has by the end of the colony spam phase (roughly 20 turns * map size) and just declare whoever has the most the winner, as they should have an insurmountable lead. The fact that they sometimes don't is usually down to AI incompetence; if a player has the most planets when there's no more left to take for free, then the only thing that can stop him is an alliance of other players with more planets.