There are a number of likely reasons why Spurs haven't joined the transfer madness this summer.
1. They are in the midst of drawing down a vast chunk of debt. And only once the stadium is built will they begin to make that debt pay. In the meanwhile, I think it highly likely that the banks lending the money will have insisted upon certain financial restrictions being met - most especially, because of professional football's poor reputation within financial circles, that transfer spend and player wages are tightly controlled. So I doubt that Spurs could make a big splash in the transfer market even if they wanted to. Once the stadium is built and the extra income is flowing into the coffers, Spurs will be able to restructure their debt and loosen the purse strings.
2. They are in something of a catch 22 situation with regard to transfers. They already have a good team. But they have limited resources with which to pay their players a sum that is still considerably less than the going rate. The players tolerate it for now because they have bought into the idea of the project that Levy and Pochettino have sold them. So almost of all them have recently signed new deals. But if Spurs were to try to compete for top quality signings with the likes of City, United, Chelsea or Arsenal, they would only stand a chance of being successful if they were to offer comparable wages. And if they did that, their other top players would want parity at least - something that Spurs cannot afford. So the very top players are therefore out of reach and players on the level below won't really improve the team but will block the path to the first team for academy products.
3. Which brings me on to the third point - Spurs have a good academy and a coach who enjoys and has a good reputation for developing young players. Harry Winks has already proved that he has what it takes to be competitive at the level to which Spurs aspire. Josh Onomah, Kyle Walker Peters and Cameron Carter Vickers all have the potential to follow him into the first team squad next season. Pochettino prefers to keep the best young players in and around the first team squad rather than send them on loan. So if that squad becomes bloated with purchased players that don't improve the first team, then opportunities will be thin on the ground for the best academy graduates - no loan and no first team appearances either. So best to keep the money and trust in Poch and youth.