There must be a balance between the incentive to support the network and the inflation that affects regular investors/users who are not staking. The major fiat currencies like USD and EUR have a target inflation rate of around 2%, and people still complain about it. As a comparison, 5% for the whole monetary mass is big. If the interest was working the way I thought it did and the stakers got 5% while the rest got zero, than the inflation rate would be only 1.66% if only one third of the coins are staked, which is much better.
Another point I want to make: setting the 5% at the overall supply level guarantees that you know exactly how supply will grow. To ensure this system-wide certainty one has to accept variations at the individual wallet level (but as mentioned previously, these fluctuations do not impact wallets negatively, as they always receive MORE not less).
If you fix the dividend at the individual staking level, it becomes impossible to accurately predict the true increase in supply.
For price / market stability the overall system-wide features are more important than those at the wallet level. Thus, even economically, this system implemented by XSPEC I find much better than those where the overall supply can fluctuate randomly.